support relative paths in musiconhold.conf, which makes moh work by default...

support relative paths in musiconhold.conf, which makes moh work by default when Asterisk was configured using --prefix and 'make samples' is run



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@149917 65c4cc65-6c06-0410-ace0-fbb531ad65f3