Move uio enums to sys/_uio.h.

Include _uio.h instead of uio.h in several headers to reduce header
polution.

Fix a few places that relied on header polution to get the uio.h header.

I have not moved struct uio as many more things that use it rely on
header polution to get other definitions from uio.h.

Reviewed by: cem, kib, markj
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14811