Add a new API function, written at least twice in app_voicemail.c

Add a new API function, written at least twice in app_voicemail.c
and likely in other places too.

This is quite useful when placing mail/html stuff in config files.

 /*!
   \brief Convert some C escape sequences (\b\f\n\r\t) into the
        equivalent characters.
   \brief s The string to be converted (will be modified).
   \return The converted string.
  */
 char *ast_unescape_c(char *s);

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@93950 65c4cc65-6c06-0410-ace0-fbb531ad65f3