use a function, set_ext_pri(), to assign extension and priority.

On passing, fix a bug in __ast_pbx_run() where the wrong priority
was used in a call to ast_exists_extension() (see comment in the code).



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@21392 65c4cc65-6c06-0410-ace0-fbb531ad65f3