bin/tests/system/stop.pl only waits for the PID file to be cleaned up
while named cleans up the lock file after the PID file.  Thus, the
aforementioned script may consider a named instance to be fully shut
down when in fact it is not.

Fix by also checking whether the lock file exists when determining a
given instance's shutdown status.  This change assumes that if a named
instance uses a lock file, it is called "named.lock".

Also rename clean_pid_file() to pid_file_exists(), so that it is called
more appropriately (it does not clean up the PID file itself, it only
returns the server's identifier if its PID file is not yet cleaned up).