pbx_config: Only the first [globals] section is seen.

If multiple [globals] sections are used (for example via separate
included files), only the first one is processed.  This can result in
lost global variables when using a modular extensions.conf.

ASTERISK-28146 #close

Change-Id: Iaac810c0a7c4d9b1bf8989fcc041cdb910ef08a0