For now, just provide the types used by sys/socket. We could
add readv() and writev() later, if desired.
* modules/sys_uio: New module.
* modules/sys_uio-tests: Likewise.
* lib/sys_uio.in.h: New file.
* m4/sys_uio_h.m4: Likewise.
* tests/test-sys_uio.c: Likewise.
* doc/posix-headers/sys_uio.texi (sys/uio.h): Document it.
* MODULES.html.sh (systems lacking POSIX:2008): Likewise.
Signed-off-by: Eric Blake <eblake@redhat.com>
