features.c: Allow appliationmap to use Gosub.

Using DYNAMIC_FEATURES with a Gosub application as the mapped application
does not work.  It does not work because Gosub just pushes the current
dialplan context, exten, and priority onto a stack and sets the specified
Gosub location.  Gosub does not have a dialplan execution loop to run
dialplan like Macro.

* Made the DYNAMIC_FEATURES application mapping feature call
ast_app_exec_macro() and ast_app_exec_sub() for the Macro and Gosub
applications respectively.

* Backported ast_app_exec_macro() and ast_app_exec_sub() from v11 to
execute dialplan routines from the DYNAMIC_FEATURES application mapping
feature.

NOTE: This issue does not affect v12+ because it already does what this
patch implements.

AST-1391 #close
Reported by: Guenther Kelleter

Review: https://reviewboard.asterisk.org/r/3844/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@419630 f38db490-d61c-443f-a65b-d21fe96a405b