The check to ensure that xstate_bv always has XFEATURE_ENABLED_X87 and
XFEATURE_ENABLED_SSE bits set is not needed.  CPU correctly handles
any bitmask which is subset of the enabled bits in %XCR0.

More, CPU instructions XSAVE and XSAVEOPT could write the mask without
e.g. XFEATURE_ENABLED_SSE, after the VZEROALL.  The check prevents the
restoration of the otherwise valid FPU save area.

In collaboration with:	jhb
MFC after:	1 week
Partially saved extended state must be handled always, i.e. for both
fpu-owned context, and for pcb-saved one.  More, the XSAVE could do
partial save, same as XSAVEOPT, so qualifier for the handler should be
use_xsave and not use_xsaveopt.

Since xsave_area_desc is now needed regardless of the XSAVEOPT use,
remove the write-only use_xsaveopt variable.

In collaboration with:	jhb
MFC after:	1 week
Add accept4() system call.

The accept4() function, compared to accept(), allows setting the new file
descriptor atomically close-on-exec and explicitly controlling the
non-blocking status on the new socket. (Note that the latter point means
that accept() is not equivalent to any form of accept4().)

The linuxulator's accept4 implementation leaves a race window where the new
file descriptor is not close-on-exec because it calls sys_accept(). This
implementation leaves no such race window (by using falloc() flags). The
linuxulator could be fixed and simplified by using the new code.

Like accept(), accept4() is async-signal-safe, a cancellation point and
permitted in capability mode.
Regenerate files for accept4().