systemd: drop RefuseManualStart=true, disable kresd.service by default

I introduced RefuseManualStart=true last year when writing the initial systemd unit file for kresd. I did this because i didn't want to start running the daemon if no one was going to talk to it.

I'm no longer convinced that this directive has a good justification. While i think that there is no need to activate the service without socket activation, i think preventing manual invocation is actively harmful (admins will reflexively do "systemctl restart kresd" just like they do "systemctl restart apache2" and it's jarring to have it rejected).

A distributor who wants to (by default) delay kresd startup until it is accessed can ship the preset file included here, but leave the *.socket units enabled by default.