transmission: improve systemd support

Support running transmission-daemon in system-wide mode using systemd:

1. Pass --with-systemd-daemon to configure script if systemd is enabled.

2. Create the transmission user if having daemon

3. Install transmission-daemon.service for systemd.

[Aurélien : Fix symlink relative path, use upstream service file,
no need for config path, and use transmission user in sysv script,
remove sysctl stuff]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Aurélien Chabot <aurelien@chabot.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>