core: skip ReadOnlyPaths= and other permission-related mounts on PermissionsStartOnly...

core: skip ReadOnlyPaths= and other permission-related mounts on PermissionsStartOnly= (#5309)

ReadOnlyPaths=, ProtectHome=, InaccessiblePaths= and ProtectSystem= are
about restricting access and little more, hence they should be disabled
if PermissionsStartOnly= is used or ExecStart= lines are prefixed with a
"+". Do that.

(Note that we will still create namespaces and stuff, since that's about
a lot more than just permissions. We'll simply disable the effect of
the four options mentioned above, but nothing else mount related.)

This also adds a test for this, to ensure this works as intended.

No documentation updates, as the documentation are already vague enough
to support the new behaviour ("If true, the permission-related execution
options…"). We could clarify this further, but I think we might want to
extend the switches' behaviour a bit more in future, hence leave it at
this for now.

Fixes: #5308