set moh category on inbound routes

we should add a music category to inbound routes (and probably direcdid routes in extensions and users) so that different music classes could be chosen as calls are routed in from different dids.

Quick test shows that the following right after setting FROM_DID works fine (the channel doesn't have to be answered first.)

{noformat}
exten => 5552231234,n,SetMusicOnHold(class){noformat}