Move grabbers definitions to a separate file, vgrabbers.c, so it is easier

Move grabbers definitions to a separate file, vgrabbers.c, so it is easier
to add more entries. This required moving struct grab_desc to the common
header, and adding an entry in the Makefile.

On passing, cleanup some comments and file headers (some are still missing).

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@95313 65c4cc65-6c06-0410-ace0-fbb531ad65f3