libc/net: Fix some issues in our inet6_opt_init() (from RFC 3542).

* The RFC says (in section 10.1) that only when extbuf is not NULL,
  extlen shall be checked, so don't perform this check when NULL is
  passed.

* socklen_t is unsigned, so checking extlen for less than zero is
  not needed.
acpi/cpu: Remove the cpux naming hack

Used to avoid name conflict between C-state module and cpu module