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
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