debian/tools: new init script

debian/tools: new init script

This separates the init script used for the system (and called in the
systemd unit file) from the script that watchfrr uses to control
daemons.  Mixing these two caused the entire thing to become a rather
huge spaghetti mess.

Note that there is a behaviour change in that the new script always
starts zebra regardless of zebra_enable.

Side changes:
- Ubuntu 12.04 removed from backports since it doesn't work anyway
- zebra is always started regardless of zebra_enable.  To disable FRR,
  the entire init script should be disabled through policy.
- no-watchfrr operation is no longer supported by the scripts in the
  Debian packages.  (This is intentional.)

Signed-off-by: David Lamparter <equinox@diac24.net>