CLEANUP: config: make parse_cpu_set() return documented values

CLEANUP: config: make parse_cpu_set() return documented values

parse_cpu_set() stopped returning the undocumented -1 which was a
leftover from an earlier attempt, changed from ulong to int since
it only returns a success/failure and no more a mask. Thus it must
not return -1 and its callers must only test for != 0, as is
documented.