MFC r231949:

MFC r231949:
Fix found places where uio_resid is truncated to int.

Add the sysctl debug.iosize_max_clamp, enabled by default. Setting the
sysctl to zero allows to perform the SSIZE_MAX-sized i/o requests from
the usermode.

MFC r232493:
Remove unneeded cast to u_int. The values as small enough to fit into
int, beside the use of MIN macro which performs type promotions.

MFC r232494:
Instead of incomplete handling of read(2)/write(2) return values that
does not fit into registers, declare that we do not support this case
using CTASSERT(), and remove endianess-unsafe code to split return value
into td_retval.

While there, change the style of the sysctl debug.iosize_max_clamp
definition.

MFC r232495:
pipe_read(): change the type of size to int, and remove signed clamp.
pipe_write(): change the type of desiredsize back to int, its value fits.

git-svn-id: svn://svn.freebsd.org/base/stable/9@233353 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f