Plinth now hosted on (server)/plinth.

Plinth has been moved from plinth.(server).local to (server)/plinth.
*plinth.py* has been updated to take a new *--server_dir* argument,
which *share/init.d/plinth* now provides.  *plinth.sample.config* has
also been updated.

Actually, the whole package has been moved to a more Debian-friendly
configuration.  *share/apache2/plinth.conf* has been updated to
reflect the standard Debian directories.  It seems to make more sense
this way, as (other than FreedomMaker, which now uses this package
anyway) no other tools or derivatives use this system.  The
configuration can be patched out by other distributions easily enough.