units: enable MaxConnectionsPerSocket= for all our Accept=yes units

units: enable MaxConnectionsPerSocket= for all our Accept=yes units

Let's make sure that user's cannot DoS services for other users so
easily, and enable MaxConnectionsPerSocket= by default for all of them.

Note that this is mostly paranoia for systemd-pcrextend.socket and
systemd-sysext.socket: the socket is only accessible to root anyway,
hence the accounting shouldn#t change anything. But this is just a
safety net, in preparation that we open up some functionality of these
services sooner or later.