Minor cleanup mostly related to proto_reg_handoff

Minor cleanup mostly related to proto_reg_handoff
 Remove code for unused handles;
 Localize handles (in proto_reg_handoff) which need not be global;
 Localize (in proto_reg_handoff) "saved prefs";
 Use find_dissector instead of create_dissector_handle as appropriate;
 Use gboolean for "initialized" flag in proto_reg_handoff.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@26693 f5534014-38df-0310-8fa8-9805f1628bb7