src/core/manager.c: log preset activity on first boot

src/core/manager.c: log preset activity on first boot

This gives us a little more information about what units were enabled
or disabled on that first boot and will be useful for OS developers
tracking down the source of unit state.

An example with this enabled looks like:

```
NET: Registered PF_VSOCK protocol family
systemd[1]: Applying preset policy.
systemd[1]: Unit /etc/systemd/system/dnsmasq.service is masked, ignoring.
systemd[1]: Unit /etc/systemd/system/systemd-repart.service is masked, ignoring.
systemd[1]: Removed '/etc/systemd/system/sockets.target.wants/systemd-resolved-monitor.socket'.
systemd[1]: Removed '/etc/systemd/system/sockets.target.wants/systemd-resolved-varlink.socket'.
systemd[1]: Created symlink '/etc/systemd/system/multi-user.target.wants/var-mnt-workdir.mount' → '/etc/systemd/system/var-mnt-workdir.mount'.
systemd[1]: Created symlink '/etc/systemd/system/multi-user.target.wants/var-mnt-workdir\x2dtmp.mount' → '/etc/systemd/system/var-mnt-workdir\x2dtmp.mount'.
systemd[1]: Created symlink '/etc/systemd/system/afterburn-sshkeys.target.requires/afterburn-sshkeys@core.service' → '/usr/lib/systemd/system/afterburn-sshkeys@.service'.
systemd[1]: Created symlink '/etc/systemd/system/sockets.target.wants/systemd-resolved-varlink.socket' → '/usr/lib/systemd/system/systemd-resolved-varlink.socket'.
systemd[1]: Created symlink '/etc/systemd/system/sockets.target.wants/systemd-resolved-monitor.socket' → '/usr/lib/systemd/system/systemd-resolved-monitor.socket'.
systemd[1]: Populated /etc with preset unit settings.
```

Considering it only happens on first boot and not on every boot I think
the extra information is worth the extra verbosity in the logs just for
that boot.