Loading a saved game fails when the save folder isn't the game folder #1628

What happens

Starting NetHack with an existing saved game shows a garbled "Who are you?"
screen with a message like:

    freopen of save/501player for compress failed; (2) No such file or directory

and the game usually crashes right after.

When

When the save directory (VAR_PLAYGROUND) is a different folder from the one
NetHack switches into at startup (HACKDIR). Packaged builds such as Homebrew
set these to different folders.

Cause

In src/files.c, plname_from_file() (un)compresses the save file using a
relative path, so it's looked up in the wrong folder. Everywhere else the
full path is built with fqname().

Possible Fix

Use fqname() for those two calls, like the rest of the save code:

nh_uncompress(fqname(gs.SAVEF, SAVEPREFIX, 0));
...
nh_compress(fqname(gs.SAVEF, SAVEPREFIX, 0));