file.c: Add ability to search custom dir for sounds

file.c: Add ability to search custom dir for sounds

To better co-exist with sounds files that may be managed by
packages, custom sound files may now be placed in
AST_DATA_DIR/sounds/custom instead of the standard
AST_DATA_DIR/sounds/<lang> directory.  If the new
"sounds_search_custom_dir" option in asterisk.conf is set
to "true", asterisk will search the custom directory for sounds
files before searching the standard directory.  For performance
reasons, the "sounds_search_custom_dir" defaults to "false".

Resolves: #315

UserNote: A new option "sounds_search_custom_dir" has been added to
asterisk.conf that allows asterisk to search
AST_DATA_DIR/sounds/custom for sounds files before searching the
standard AST_DATA_DIR/sounds/<lang> directory.

(cherry picked from commit 0e0f99db1d62b7642a51d2552256eece5d435bc8)