postgresql: add systemd unit file

Add systemd unit file for postgresql.
When 'sysvinit' and 'systemd' are both in DISTRO_FEATURES, we need to prevent
the init script from running via systemd.

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>