pcap_io: Only check byteswapped linktype if pcap magic is swapped

In pcap_validate_header() the linktype and the byte-swapped linktype are
currently checked against the supported linktypes. Since the swapped
linktype is always larger than LINKTYPE_MAX, only one of the two tests
was actually done. Make this intention a bit more clear by explicitly
checking only for either the swapped or non-swapped linktype, depending
on whether the pcap magic is swapped.

Also make the error messages a bit more verbose regarding the
major/minor version.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>