document systemd::unit_file example with puppet-strings

* `systemd::manage_coredump` to enable everything
* `systemd::coredump_settings` will manage `/etc/systemd/coredump.conf`
* `systemd::backtrace` if true will add --backtrace to systemd-coredump
   call

So typical use would be

```puppet
class{'systemd':
  manage_coredump   => true,
  coredump_settings => {
    'Storage' => 'external',
  },
  coredump_backtrace => true,
}
```

to enable storing cores on disk and writing backtraces to
the journal.

Manage systemd-coredump config and setup

Without this, issues like:
 cvmfs-fsck.timer: Refusing to start, unit cvmfs-fsck.service to trigger not loaded.
may be observed.

timer: timer unit must depend on service unit.

MR #246 started installing the `systemd-resolved` package
on RedHat 8 and 9 however this quite wrong for 8 where
systemd-resolved is not a sub package.

```
> rpm -qf /usr/bin/systemd-resolve
systemd-239-56.el8.x86_64
```

Do not install  systemd-resolved RedHat 8

If the parameter is true (default) then `/etc/resolv.conf` will
be managed when `manage_resolved` is true.

New parmater manage_resolv_conf for /etc/resolv.conf

Addition of Trivial Acceptance Tests

If `systemd-resolved` is stopped but managed via

```
class{'systemd:
  managed_resolved => true,
  resolved_ensure  => 'stopped',
}
```
Then the files in `/var/run/systemd/resolv` will either
be stale(just stopped) or missing(after reboot). In both cases it
makes no sense to manage `/etc/resolv.conf`.

More over for the case where `systemd-resolved` has just
been stopped reinstate `/run/systemd/resolve/resolv.conf`
as `/etc/resolv.conf`.

Don't manage /etc/resolv.conf if systemd-resolved is stopped

Following #246 and #254 systemd-resolved was installed on OS family
RedHat 9 only.

On RedHat 7 a systemd-resolved package is now installed as it is
available.

For information it is only RedHat 8 that does not have a systemd-resolved
package.

Install systemd-resolved on RedHat 7

Release 3.7.0