- Zero out the MSW of %rsi "just in case".
- Cleanup ABI translation for syscall -> cpu_fetch_syscall_args()

Approved by: gibbs (implicit)
- We don't support Process Context Identifiers (PCID) (the x86 version of ASIDs) yet.
- You've got to flush that TLB when you've got to flush that TLB (as
  soon as the new user pmap has been loaded).
- Do not destroy the pmap lock when a pmap is destroyed. The zone
  .init()/.fini() lock handling is leaky.
- Remove stray #include

These changes now enable fork(2) to succeed, and both parent and child
to make progress.

Next stop, exec(2)

Approved by: gibbs(implicit)