Move AST_FEATURE_FLAG_* and FEATURE_RETURN_* to features.h so that they can be used by modules.

(closes issue #12384)
Reported by: fnordian
Patches:
      features.patch uploaded by fnordian (license 110)

(patch modified by me, to give FEATURE_RETURN_* an AST_ prefix)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@113452 65c4cc65-6c06-0410-ace0-fbb531ad65f3