Provide a systemd service file in the source tree, but do not install it

Provide a systemd service file in the source tree, but do not install it

This endeavours to do what the sysvinit file does - create
/var/run/userv iff necessary and start uservd -daemon.

Matthew's original patch in Debian #939573 also shipped this file in
the .deb.  I'm not sure what the benefit of that would be and it seems
to add more test complications, so for now I am leaving that part out.

Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>