Write out decode_as entries from the actual dissector tables.

Have a save_decode_as_entries() routine in ui/decode_as_utils.c, that
does all the work of saving the entries by iterating through all the
changed entries with dissector_all_tables_foreach_changed().

When doing so, write out the selector for integral values in decimal, as
older versions of Wireshark only handled decimal values, and some of
those older versions are EOLed and won't be fixed.

Change-Id: I2dab461604524b98e3515867839a4b86c86c5d7b
Reviewed-on: https://code.wireshark.org/review/3426
Reviewed-by: Guy Harris <guy@alum.mit.edu>