man: drop references to "cgroup" wher appropriate

Since cgroups are mostly now an implementation detail of systemd lets
deemphasize it a bit in the man pages. This renames systemd.cgroup(5) to
systemd.resource-control(5) and uses the term "resource control" rather
than "cgroup" where appropriate.

This leaves the word "cgroup" in at a couple of places though, like for
example systemd-cgtop and systemd-cgls where cgroup stuff is at the core
of what is happening.