Have wsutil/exported_pdu_tlvs.h define the LINKTYPE_WIRESHARK_UPPER_PDU
TLV type and length values, as well as the port type values written to
files in EXP_PDU_TAG_PORT_TYPE TLVs.
Update the comment that describes the LINKTYPE_WIRESHARK_UPPER_PDU TLVs
to more completely and correctly reflect reality (it was moved from
epan/exported_pdu.h to wsutil/exported_pdu_tlvs.h).
Rename those port type values from OLD_PT_ to EXP_PDU_PT_; there is
nothing "old" about them - yes, they originally had the same numerical
values as the PT_ enum values in libwireshark, but that's no longer the
case, and the two are now defined independently. Rename routines that
map between libwireshark PT_ values and EXP_PDU_PT_ values to remove
"old" from the name while we're at it.
Don't include epan/exported_pdu.h if we only need the
LINKTYPE_WIRESHARK_UPPER_PDU definitions - just include
wsutil/exported_pdu_tlvs.h.
In extcap/udpdump.c, include wsutil/exported_pdu_tlvs.h rather than
defining the TLV types ourselves.