Make "make-reg-dotc" generate a "register_all_protocol_handoffs()"

Make "make-reg-dotc" generate a "register_all_protocol_handoffs()"
routine, which calls all routines found in the dissector source files
with names that match " proto_reg_handoff_[a-z_0-9A-Z]*".

Call "register_all_protocol_handoffs()" after calling
"register_all_protocols()" - "register_all_protocols()" needs to be
called first, so that all protocols can register their fields, because
registering a dissector as being called if field "proto.port" is equal
to N requires that "proto.port" be a registered field.

Give DNS a handoff registration routine, and register its dissector to
be called if "udp.port" is UDP_PORT_DNS; remove the registration of DNS
from "packet-udp.c", and make "dissect_dns()" static (as nobody else
need know that it exists).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1788 f5534014-38df-0310-8fa8-9805f1628bb7