Base directory for MOH should be ASTDATADIR

Base directory for MOH should be ASTDATADIR

If the directive 'directory' is relative, make it relative to the
datadir, rather than to the varlibdir. In the sample configuration
it is relative ('moh').

This has no effect unless you have actively set the datadir explicitly
(at build time or at run time).

(closes issue #16906)
Patches:
      moh_datadir uploaded by tzafrir (license 46)

Review: https://reviewboard.asterisk.org/r/974/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.2@292049 65c4cc65-6c06-0410-ace0-fbb531ad65f3