seafile-server: fix fastcgi mode settings in seafile-admin script

* Use the correct path for the pidfile
* Start a multithreaded daemon instead of pre-forking worker processes.
  This avoids consuming too much memory and makes context switches faster

Signed-off-by: Gergely Kiss <mail.gery@gmail.com>
Merge pull request #691 from kissg1988/seafile

seafile-server: fix fastcgi mode settings in seafile-admin script