Use a dynamically sized array to store the list of files for moh "files" mode

 - Instead of always allocating 64KB of memory for every MOH class, this has
   been reduced to only a single pointer per class, with more memory only
   allocatted when using "files" mode, as needed
 - Instead of imposing a length limit on the full filename, including full
   path, of 127 characters, use PATH_MAX, the maximum length that the system
   can handle
 - There is no longer a limit on the number of files than can be used for a
   single MOH class using "files" mode


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@31953 65c4cc65-6c06-0410-ace0-fbb531ad65f3