* pid_output.c: (pid_output_lock) Fix 2 bugs: when locking, should
set l_whence to SEEK_SET, not SEEK_END. And after writing new
pid to file, must ftruncate to eliminate any extraneous bytes left
over from the last time a pid was written.
* lib/pid_output.c: (pid_output) call pid_output_lock if we have
fcntl().
(pid_output_lock) grab exclusive write lock on pid file, rather
than rely on (fragile) exclusive create.