units: remove RefuseManualStart from units which are always around

In a normal running system, non-passive targets and units used during
early bootup are always started. So refusing "manual start" for them
doesn't make any difference, because a "start" command doesn't cause
any action.

In early boot however, the administrator might want to start on
of those targets or services by hand. We shouldn't interfere with that.

Note: in case of systemd-tmpfiles-setup.service, really running the
unit after system is up would break the system. So e.g. restarting
should not be allowed. The unit has "RefuseManualStop=yes", which
prevents restart too.
doc: grammatical corrections
doc: typographical improvements and choice of words
doc: use expanded forms for written style