[telepathy-sasl-signon] Use booster for launching

libsailfishkeyprovider is a privileged resource, so we need to launch
via the booster to gain privileges. Some careful systemd work related to
mission-control-5 will be done to make sure that booster-generic is
alwyas available when necessary.
Merge pull request #14 from special/use-booster

[telepathy-sasl-signon] Use booster for launching