Partial MFC:
implement kernel side of i386_set_gsbase(2) and friends.
Add symbol to tell NDIS which slot to use.
Notable differences to -current:

• gdt renumber not merged
• user %cs/%ds still live in the ldt.
• NDIS_SEL defined to same value as ndis code had hardcoded(!).

We could have libc and rtld switch to using this as the primary API
to implement TLS and avoid having a custom LDT for every process.