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: http://svn.asterisk.org/svn/asterisk/trunk@399750 f38db490-d61c-443f-a65b-d21fe96a405b