biology/linux-foldingathome: Add manual page fahclient(8) and clean up

Move all documentation to fahclient.8.

Reference the new manual page from pkg-message and the rc script.

Remove outdated --chdir handling. The flag is gone from fah-client and
rc(8) handles that automatically.

Remove an incorrect description of what fahclient_flags does.

Allow fahclient_team and fahclient_donor to be empty. This way we avoid
warnings when the configuration file overrides the command-line flags,
e.g.:

14:25:24:W :Option 'user' already set to 'Anonymous' reseting to '0mp'.
14:25:24:W :Option 'team' already set to '11743' reseting to '11743'.

Pass --log-to-screen=true to showconfig's flags. This way we can
see again the configuration.

Improve quoting of variables in fahclient.

Update to the v8.5 branch is currently blocked by some missing
libraries. It seems that we would need to add a Rocky Linux 9.7 version
of bzip2 to proceed.