package/postgresql: change systemd service type to notify

package/postgresql: change systemd service type to notify

During activation, pg_ctl uses exec to start the db server, which causes
the service to never finish activating when Type=forking. Upstream
recommends configuring --with-systemd and using Type=notify.

https://www.postgresql.org/docs/10/static/server-start.html

Upstream says:
    When using systemd, you can use the following service unit file
    [...]
    Using Type=notify requires that the server binary was built with
    configure --with-systemd.

Signed-off-by: Joseph Kogut <joseph.kogut@gmail.com>
Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>