core/pbx: dialplan show - display filename/line#

Adds the ability for extensions to be registered to include filename and
line number so that dialplan show output can show the filename and line
number of a config file responsible for generating a given extension.

This only affects config modules that are written to use the new extension
registering functions. In this patch, that only includes pbx_config, so
extensions registered in extensions.conf and any included extension will
be shown in this manner. Extensions registered in this manner will show
the filename and line number *instead* of the registrar.

ASTERISK-26658 #close
Reported by: Jonathan R. Rose

Change-Id: Ieccc6abccdff34ed5c7da3511fd24972b8f2dd30