Have "wtap_read()" set "wth->phdr.pkt_encap" to "wth->file_encap",

Have "wtap_read()" set "wth->phdr.pkt_encap" to "wth->file_encap",
rather than requiring individual capture file type handlers to do it
(unless they're doing per-packet encapsulation, in which case we check
to make sure they didn't *leave* it as WTAP_ENCAP_PER_PACKET).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@10290 f5534014-38df-0310-8fa8-9805f1628bb7