Fix skinny to recognise vmexten in general section of conf

Fixup the vmexten so if globally set in general section will be honored by
chan_skinny. Also get rid of the 'global_' part of variable name to match
regexten.

(closes issue ASTERISK-20790)
Reported by: snuffy
Tested by: snuffy, myself
Patches: 
    skinny-vm.diff uploaded by snuffy (license 5024)
........

Merged revisions 378010 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378011 65c4cc65-6c06-0410-ace0-fbb531ad65f3