logind: rename EnableWallMessages= configuration knob to WallMessages= (#37553)

logind: rename EnableWallMessages= configuration knob to WallMessages= (#37553)

We generally frown on config options that are called
EnableXYZ=/DisableXYZ=, hence drop this here too. Note that the original
sin was that the pre-existing D-Bus property already used the prefix.
But given that D-Bus properties are substantially more low-level (i.e.
developer focused) than the config knobs, let's bite the bullet and
name the configuration option cleanly, even if the dbus property carries
the damn prefix.

The knob was added post v257, hence this is not a compat break.

Follow-up for: 130698dc20df9438c3f5b79ddf2c1a1b16c3f8fc