Linux: Get DLT_LINUX_DSA_UNKNOWN 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_LINUX_DSA_UNKNOWN;
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_LINUX_DSA_UNKNOWN, 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