pid1: unify implemenation of /run/ disk space safety check a bit

pid1: unify implemenation of /run/ disk space safety check a bit

reload/reexec currently used a separate implementation of the /run/ disk
space check, different from the one used for switch-root, even though
the code is mostly the same. The one difference is that the former
checks are authoritative, the latter are just informational (that's
because refusing a reload/reexec is relatively benign, but refusing a
switch-root quite troublesome, since this code is entered when it's
already "too late" to turn turn back, i.e. when the preparatory
transaction to initiate the switch root are already fully executed.

Let's share some code, and unify codepaths.

(This is preparation for later addition of a "userspace reboot" concept)

No change in behaviour, just refactoring.