systemctl: when removing enablement or mask symlinks, cover both /run and /etc

'systemctl disable --runtime' would disable a unit, but only if it was enabled
with '--runtime', and silently do nothing if the unit was enabled persistently.
And similarly 'systemctl disable' would do nothing if the unit was enabled in
/run. This just doesn't seem useful.

This pathch changes enable/disable and mask/unmask to be asymmetrical. enable
and mask create symlinks in /etc or /run, depending on whether --runtime was
specified. disable and unmask remove symlinks from both locations. --runtime
cannot be specified for the disable and unmask verbs.

The advantage is that 'disable' now means that the unit is disabled, period.
And similarly for 'unmask', all masks are removed.

Similarly for preset and preset-all, they now cannot be called with --runtime,
and are asymmetrical: when they enable a unit, symlinks are created in /etc.
When they disable a unit, all symlinks are nuked.

$ systemctl --root=/ enable bluetooth
Created symlink /etc/systemd/system/dbus-org.bluez.service → /usr/lib/systemd/system/bluetooth.service.
Created symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service → /usr/lib/systemd/system/bluetooth.service.
$ systemctl --root=/ --runtime enable bluetooth
Created symlink /run/systemd/system/dbus-org.bluez.service → /usr/lib/systemd/system/bluetooth.service.
Created symlink /run/systemd/system/bluetooth.target.wants/bluetooth.service → /usr/lib/systemd/system/bluetooth.service.
$ systemctl --root=/ disable bluetooth
Removed /run/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed /run/systemd/system/dbus-org.bluez.service.
Removed /etc/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed /etc/systemd/system/dbus-org.bluez.service.
$ systemctl --root=/ disable --runtime bluetooth
--runtime cannot be used with disable

$ systemctl --root=/ mask --runtime bluetooth
Created symlink /run/systemd/system/bluetooth.service → /dev/null.
$ systemctl --root=/ mask bluetooth
Created symlink /etc/systemd/system/bluetooth.service → /dev/null.
$ systemctl --root=/ unmask bluetooth
Removed /run/systemd/system/bluetooth.service.
Removed /etc/systemd/system/bluetooth.service.
$ systemctl --root=/ unmask --runtime bluetooth
--runtime cannot be used with unmask

$ systemctl --root=/ --runtime enable bluetooth
Created symlink /run/systemd/system/dbus-org.bluez.service → /usr/lib/systemd/system/bluetooth.service.
Created symlink /run/systemd/system/bluetooth.target.wants/bluetooth.service → /usr/lib/systemd/system/bluetooth.service.
$ systemctl --root=/ enable bluetooth
Created symlink /etc/systemd/system/dbus-org.bluez.service → /usr/lib/systemd/system/bluetooth.service.
Created symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service → /usr/lib/systemd/system/bluetooth.service.
$ systemctl --root=/ preset bluetooth
Removed /run/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed /run/systemd/system/dbus-org.bluez.service.
Removed /etc/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed /etc/systemd/system/dbus-org.bluez.service.
$ systemctl --root=/ preset --runtime bluetooth
--runtime cannot be used with preset

$ systemctl preset-all --runtime
--runtime cannot be used with preset-all