systemd-systemctl: support instance expansion in WantedBy

Refactor _process_deps to expand systemd instance specifier "%i" to the
template instance.

This change expands on prior commit e510222b57 ("systemd-systemctl: fix
instance template WantedBy symlink construction") by substituting every
"%i" pattern-match with the instance name.

The regexp handles the following cases:

* svc-wants@%i.service

* sys-subsystem-net-devices-%i.device

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 93562761)
Signed-off-by: Steve Sakoman <steve@sakoman.com>