Commit some cleanups to the format type code.

 - Remove the AST_FORMAT_MAX_* types, as these are consuming 3 out of our available 32 bits.
 - Add a native slin16 type, so that 16kHz codecs can translate without losing resolution.
   (This doesn't affect anything immediately, until another codec has wb support.)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89071 65c4cc65-6c06-0410-ace0-fbb531ad65f3