Replace init scripts with systemd unit files

Replace init scripts with systemd unit files

The package installs four unit files. gigi-standalone.service works just
like the old cacert-gigi service: gigi will start as root, manage its
own ports, then drop privileges. gigi-proxy.service and .socket let
systemd manage the port and start gigi as its dedicated user. These
services need different configuration for gigi: for the proxy version,
the configuration must contain proxy=true and http.bindPort=stdin, while
for the standalone version the configuration must have proxy=false and
specify real ports. For this reason, we also disable Debian's policy to
automatically start services upon package installation.

(gigi-simple-signer.service is a direct conversion of
cacert-gigi-signer.init.)

Very simple init scripts for gigi-standalone and gigi-simple-signer are
provided, so that running /etc/init.d/gigi-standalone start will still
work. The scripts simply redirect to systemctl; the LSB header is not
included, since the scripts are useless on their own.

Change-Id: I53f0c825880d1b8c082496106a018957d6128392