Document socket options and minimum Kernel version required

[jwbensley]

AF_PACKET socket require Linux 2.2.

PACKET_RX_RING requires Linux 2.6.5.
PACKET_VERSION with PACKET_RX_RING requires Linux 2.6.27.
TPACKET_V2 requires Kernel 2.6.27.
PACKET_STATISTICS requires TPACKET_V2.
PACKET_TX_RING requires Linux 2.6.31.
PACKET_LOSS requires Linux 2.6.31.
PACKET_TIMESTAMP with PACKET_RX_RING requires Linux 2.6.36.
PACKET_FANOUT requires Linux 3.1.
PACKET_QDISC_BYPASS requires Linux 3.14.

PACKET_VERSION requires Linux 3.0.
PACKET_MMAP TPACKET_V2 requires Linux 2.6.17.
PACKET_MMAP TPACKET_V3 requires Linux 3.2.
TPACKET_V3 and PACKET_TX_RING together requires Linux 4.11.

[jwbensley]

Split this into minimum required (e.g. all 2.x) and optional (e.g. >= 3.x).

[jwbensley]

Ubuntu test box fails to detect it's Kernel version is too low for AF_PACKET v3 on Tx (Rx works):

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.6 LTS
Release:	14.04
Codename:	trusty

$ uname -a
Linux htpc-ubuntu 3.13.0-170-generic #220-Ubuntu SMP Thu May 9 12:40:49 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

$ sudo ./etherate_mt -i eth2 -p4
Using inteface eth2 (4).
Setting interface promiscuous mode
Frame size set to 1514 bytes.
Using raw socket with PACKET_MMAP and TX/RX_RING v3.
Running in Tx mode.
18116:Trying to increase to 1048576 bytes...
18116:Write buffer size set to 425984 bytes
18116:Can't enable Tx/Rx ring for socket (Invalid argument)
Removing interface promiscuous mode

$ date
Mon Nov 18 08:09:08 GMT 2019