logind: fix internal types used for EnableWallMessages

logind: fix internal types used for EnableWallMessages

The value is used as a boolean, but stored in an unsigned. It's exposed
with sd-bus default boolean handling which however expects a (signed)
int. Let's clear this up, and use "bool" for this everywhere in
non-local scope, and make sure that when we pass it to sd-bus we
properly pass an "int".