Allow xcb_send_request with >MAX_IOV iovecs

This allows an application to do a scatter/gather operation on a large
image buffer to avoid the extra memcpy.

Use autoconf to use UIO_MAXIOV where IOV_MAX is not available (and the
POSIX minimum of 16 where neither are available).

Reviewed-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Peter Harris <pharris@opentext.com>