man/daemon: s/init system/service manager/

man/daemon: s/init system/service manager/

As pointed out in the review, all this applies to the user services too, so are
not managed by the "init system", but by the more generic "service manager".

Also:
- use oxford comma
- change "employ" to "use" in various places
- change "the init system forwards messages to syslog" to "are forwarded to
  syslog". This is done by systemd-journald, so really there is no forwarding,
  because systemd-journald just writes them to a file in the common setup,
  so let's use the passive form to avoid specifying who does this.