Linux: Get DLT_DEBUG_ONLY and DLT_EN10MB right. [skip appveyor]

In dsa_protos[] remove the "none" non-DSA case to make the array purely
DSA and switch all DLT_EN10MB DSA tags types to DLT_DEBUG_ONLY; update
the comments to make it clear that using DLT_EN10MB would not work, give
better directions for what to do instead, do not say for every DSA tag
whether a DLT is/isn't and should/shouldn't be assigned because that is
now supposed to be obvious, do not suggest DLT_LINUX_SLL (this would add
the packet direction at the cost of losing other headers).

In iface_dsa_get_proto_info() handle the "none" non-DSA case first; for
a DSA case default to DLT_DEBUG_ONLY, always return 1, make sure the DLT
is never DLT_EN10MB and add a comment to explain the rationale.

See also GH the-tcpdump-group#1367 and the-tcpdump-group#1451.

Author: infrastation