Explicitly disable RunAsDaemon in the systemd unit file (#12731).

Our current systemd unit uses "Type = simple", so systemd does not expect tor to
fork. If the user has "RunAsDaemon 1" in their torrc, then things won't work as
expected. This is e.g. the case on Debian (and derivatives), since there we pass
"--defaults-torrc /usr/share/tor/tor-service-defaults-torrc" (that contains
"RunAsDaemon 1") by default.

The only solution I could find is to explicitly pass "--RunAsDaemon 0" when
starting tor from the systemd unit file, which this commit does.
Verify configuration file via ExecStartPre in the systemd unit file (#12730).
Merge remote-tracking branch 'intrigeri/bug12730-systemd-verify-config' into maint-0.2.5
Merge remote-tracking branch 'intrigeri/bug12731-systemd-no-run-as-daemon' into maint-0.2.5

Conflicts:
	contrib/dist/tor.service.in
Merge remote-tracking branch 'origin/maint-0.2.5'