astobj2: Made use OBJ_SEARCH_xxx identifiers as field enum values internally.

* Made ao2_unlink to protect itself from stray OBJ_SEARCH_xxx values
passed in.
........

Merged revisions 399749 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399750 65c4cc65-6c06-0410-ace0-fbb531ad65f3