Mark nscd service as forking in systemd service file (BZ #16639)

Mark nscd service as forking in systemd service file (BZ #16639)

Currently the nscd service is installed in systemd as a simple
service, which means that it is able to handle its own errors and does
not quit.  Since nscd does not fit that description, i.e. it can exit
on errors like, say, failing to parse nscd.conf, it should be declared
as forking instead.