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Change type
  New feature, not for 3GPP MBMS use. This branch is the FLUTE version 2 delta and nothing else.

Basis
  The two versions are separate protocols, not a compatible upgrade.

  RFC 3926 clause 3.4.1: "This document specifies FLUTE version 1. Hence in any ALC packet that
  carries FDT Instance and that belongs to the file delivery session as specified in this
  specification MUST set this field to '1'."

  RFC 6726 clause 11.1: "Therefore, an implementation that relies on [RFC3926] and RFC 3451 will
  not be backwards compatible with FLUTE as specified in this document."

  RFC 6726 clause 3.1, which is why the receive side checks against a configured value rather than
  accepting whatever arrives: "If multiple FLUTE sessions are sent to a channel, then receivers
  MUST determine the FLUTE protocol version, based on version fields and the (source IP address,
  TSI) pair carried in the ALC/LCT header of the packet."

  On the LCT generation, RFC 6726 clause 11.1: "In [RFC5651], these fields MUST be set to zero and
  MUST be ignored by receivers (instead, the EXT_TIME Header Extensions can convey this
  information if needed)."

  On the identifier sequence, RFC 6726 clause 3.4.1: "After reaching the maximum value (2^20-1),
  the numbering starts from the smallest FDT Instance ID value assigned to an expired FDT
  Instance." and "Senders MUST NOT reuse an FDT Instance ID value that is already in use for a
  non-expired FDT Instance."

  On expiry, RFC 6726 clause 3.3: "both a sender and a receiver easily determine to which
  (136-year) epoch the FDT Instance expiration time value pertains by choosing the epoch for which
  the expiration time is closest in time to the current time."

  On a shared Content-Location, RFC 6726 clause 3.4.2: "the element appearing in the FDT Instance
  with the greater FDT Instance ID is considered to declare a newer instance (e.g., version) of
  the same"

  On security, RFC 6726 clause 7.5 takes its service set from ALC, where RFC 5775 requires
  anti-replay support alongside authentication and integrity.

Change
  A per-session FLUTE version, settable on the Transmitter and Receiver, defaulting to 1, carried
  into the ALC parser, the FDT and the receiver's object handling. Every behavioural difference
  below is reached only when a caller selects version 2, so a version 1 or 3GPP session is
  unchanged in every respect.

  Signalling: transmit writes the selected version into the EXT_FDT nibble, receive accepts only
  the configured version, and the RFC 6726 FDT namespace is enabled.

  LCT generation: under version 2 the two bits RFC 3451 used for the Sender Current Time and
  Expected Residual Time fields contribute no header words and nothing is stepped over, so an
  extension placed after the TOI is reached. Under version 1 the RFC 3451 reading is untouched.
  Transmit has always sent both bits zero, since the header buffer is calloc'd.

  FDT Instance ID: version 2 wraps to the smallest expired identifier, refuses one still live, and
  reports exhaustion, which RFC 6726 clause 3.4.1 leaves to the implementation. Version 1 keeps
  RFC 3926's wrap to zero.

  Expiry: read into the era nearest the current time, version 2 only.

  Shared Content-Location: the receiver records which instance declared each file and keeps the
  one from the greater instance ID rather than whichever completed first, version 2 only.

  IpSec sets a replay window. That one is version-independent and benefits both.

  Four further obligations were checked against the specifications and found already met, so no
  code changed for them: EXT_TIME and EXT_AUTH recognition, the RFC 3450 to RFC 5775 move, the FDT
  schema body, and ESP authentication and integrity. README-FLUTE-V2.md records each, how it was
  established, and where the reading is weaker than a test.

  The default is what keeps this safe: 1 is the version TS 26.346 clause L.4.1 references, so a
  session that does not ask for version 2 behaves exactly as it does on the version 1 branch.

Separation, deliberate
  Rebuilt on the FLUTE version 1 compliance branch rather than on the FEC branches, so it carries
  no Raptor and no RaptorQ: zero files of either. The Raptor and RaptorQ test suites are
  deliberately not registered in tests/CMakeLists.txt here, and the reason is stated at that line.

Verification
  T1: 62 cases pass at the build root, 12 of them in the FluteV2 suite, covering both readings of
  the LCT bits, the three branches of the version 2 identifier sequence, and the worked example
  RFC 6726 clause 3.3 gives for the era calculation. Quotations checked with
  tools/verify-citations.py in --strict mode.
  T2: ip xfrm state show on an installed association reports "replay-window 32" beside the enc and
  auth-trunc lines. 32 is the ceiling the legacy attribute expresses, the kernel holding its replay
  bitmap in a __u32; a request for 64 is clamped.

Not in this change
  Congestion control, which RFC 5775 requires for every packet in a session and which this library
  does not implement. It is a building block, not a patch, and until it exists no RFC 6726
  conformance is claimed for this branch and none can be. It is the only remaining item.