Add an 'sms' option to mobile.conf to manually enable or disable SMS support.

(closes issue #15071)
Reported by: ughnz
Patches:
      optional-sms1.diff uploaded by mnicholson (license 96)
Tested by: ughnz, mnicholson


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@209993 65c4cc65-6c06-0410-ace0-fbb531ad65f3