asterisk.h: Add ASTERISK_REGISTER_FILE macro

asterisk.h: Add ASTERISK_REGISTER_FILE macro

The 11/13 branches and master use 2 different file version macros. 11/13
uses ASTERISK_FILE_VERSION but master uses ASTERISK_REGISTER_FILE. This
means a new file added to 11/13 can't just be cherry-picked to master
because the macro has to be changed.

To make cherry-picking possible, ASTERISK_REGISTER_FILE was added
to asterisk.h as a simple alias for ASTERISK_FILE_VERSION(__FILE__, NULL)
The "$Revision$" tag doesn't do anything since Asterisk moved to git so
just passing NULL as the verison works fine.  asterisk.h was also
annotated to deprecate ASTERISK_FILE_VERSION and suggest using
ASTERISK_REGISTER_FILE for all new files.

Finally, 2 recent file additions, pbx_builtins.c and pbx_functions.c,
were modified to use the new macro to make sure it actually worked.
'core show file version' showed the correct output.

Change-Id: I5867ed898818d26ee49bb6e5c7d4c1a45d4789a5