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
brooks