Rename port/thread.c to port/user.c.

Rename port/thread.c to port/user.c.

Historically this module dealt with thread-safety of system interfaces,
but now all that's left is wrapper code for user name and home directory
lookup.  Arguably the Windows variants of this logic could be moved in
here too, to justify its presence under port.  For now, just tidy up
some obsolete references to multi-threading, and give the file a
meaningful name.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKGLtmexrpMtxBRLCVePqV_dtWG-ZsEbyPrYc%2BNBB2TkNsw%40mail.gmail.com