Would-be fix for bug 7733: learn bridge ID from descriptor

If somebody has configured a client to use a bridge without setting
an identity digest (not recommended), learn the identity digest from
whatever bridge descriptor we have downloaded or have in our cache.
systemd unit file: set up /var/run/tor as writable for the Tor service.

For some strange reason, this was not needed with systemd v208.
But it's needed with systemd v215 on current Debian sid, and entirely
makes sense.
Add changes file for #13196.

Note that this will likely need to be folded with the changes file for #12751,
as this change is a mere fixup on top of the changes introduced for #12751.
Merge remote-tracking branch 'intrigeri/bug13196-systemd-writable-run-directory'
Merge remote-tracking branch 'public/bug7733a'
changes file for 7733