pid1: rework handling of m->show_status

The fact that m->show_status was serialized/deserialized made impossible any
further customisation of this setting via system.conf. IOW the value was
basically always locked unless it was changed via signals.

This patch reworks the handling of m->show_status but also makes sure that if a
new value was changed via the signal API then this value is kept and preserved
accross PID1 reexecuting or reloading.

Note: this effectively means that once the value is set via the signal
interface, it can be changed again only through the signal API.