Implement the '!' negation element to negate codecs directly in the allow keyword.

This permits the list of codecs to be specified in one configuration line,
instead of two or more, generally with the aim of either allowing all codecs
with the exception of a few or disallowing most but permitting a few.

Review: https://reviewboard.asterisk.org/r/1411/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@334574 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/sip.conf.sample

@@ -1216,10 +1216,14 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
         allow=gsm
         allow=g723
         allow=ulaw
+        ; Or, more simply:
+        ;allow=!all,ilbc,g729,gsm,g723,ulaw
 
 [ulaw-phone](!)                   ; and another one for ulaw-only
         disallow=all
         allow=ulaw
+        ; Again, more simply:
+        ;allow=!all,ulaw
 
 ; and finally instantiate a few phones
 ;