This is a proof of concept. I did not test it thoroughly. It boots to
multiuser without apparent issue.
This was inspired by D27016 / enables a better version of that change.
fdcopy_remapped / fdescfree_remapped are only used by CloudABI-exclusive
routines, so I did not bother to mirror those in this change, under the
assumption that CloudABI is dead.
(Also, I did not fix the userspace build at all yet. Things like libprocstat are broken.)