tlsdate-dbus-announce: use name and flush connection

Have tlsdate-dbus-announce grab a well-known bus name before it sends its
signal, and have it wait for the signal to actually be sent over the bus before
exiting.

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
Merge pull request #79 from elly/dbus-flush

tlsdate-dbus-announce: use name and flush connection