Fix skinny encall button to not blind xfer.

The softbutton endcall should not turn a transfer into a blind transfer but
hangup the exten being called and leave the original call on hold. This does
that.

(closes issue ASTERISK-21321)
Reported by: wedhorn
Tested by: snuffy, myself
Patches: 
    skinny-xferendcall01.diff uploaded by wedhorn (license 5019)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@383948 f38db490-d61c-443f-a65b-d21fe96a405b