nut: fix setting RUNAS and STATEPATH in hotplug scripts

nut: fix setting RUNAS and STATEPATH in hotplug scripts

A previous PR missed updating the hotplug scripts for the
new find_runas and find_statepath functions which emit
the value on stdout instead of setting a variable in the
caller's scope. Update that usage and handle error conditions.

Also ensure we fully handle finding the group of the RUNAS user for
the serial usb hotplug case, and handle error conditions for that.

Signed-off-by: Daniel F. Dickinson <redacted>