Prefer to call "rl_filename_completion_function" and
"rl_completion_matches", rather than using the names without the rl_
prefix.  This matches Readline's documentation, and makes our code
a little clearer about which names are external.  On platforms that
only have the un-prefixed names (just some very ancient versions of
libedit, AFAICT), reverse the direction of the compatibility macro
definitions to match.

Also, remove our extern declaration of "filename_completion_function";
whatever libedit versions may have failed to declare that are surely
dead and buried.

Discussion: https://postgr.es/m/23608.1576248145@sss.pgh.pa.us