Make pbx_config.c use Gosub instead of Macro call for stdexten.

Make pbx_config.c use Gosub instead of Macro call for stdexten.

Users created by users.conf with hasvoicemail=yes have been documented as
using a Gosub to stdexten since v1.6.0.  However, the code still generates
dialplan to access stdexten as a Macro as documented in v1.4; which does
not work with the newer extensions.conf.sample file.

* Make generated dialplan access the stdexten dialplan with the documented
Gosub instead of the older Macro style.

(closes issue ASTERISK-18809)
Reported by: Jay Allen
Patches:
      gosub_patch-pbx_config.patch (license #6323) patch uploaded by Jay Allen (modified)
Tested by: rmudgett

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@349782 65c4cc65-6c06-0410-ace0-fbb531ad65f3