Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't include sys/user.h merely for its side-effect of recursively
including other headers.

Add an assertion that the pcb_nsaved field of the pcb be less than
MAXWIN to the register window manipulation functions - rwindow_load()
calls rwindow_save() so this one addition should take care of both.
This should help find places that pcb_nsaved doesn't get initialized
properly.

Suggested by: jake

Greatly improve readability of trap() by using a table to convert between
trap types and signals to send. Rearrange KASSERTs to better handle faults
early before curthread is setup, or in the case that it gets corrupted or
set to 0.

Check the alignment of the stack pointer before copying in windows from the
user stack in response to a failed window fill, allowing the process to be
killed if its wrong. This caused user programs which misalign their stack
pointer to get stuck in an infinite loop at the kernel-userland boundary,
which is mostly harmless.

The same thing causes a fatal RED state exception on OpenBSD and probably
NetBSD.

Inspired by: art@openbsd.org

Add a macro for shift of an integer (1 << shift == sizeof). Move the pointer
define to live alongside it. For kicks assert at compile time that they are
correct. Use these instead of magic numbers.

Catch up to change in compile time assertion interface.

Assert at compile time that structures which need to be a power of
2 really are.
Move a trace to before flushw in case it goes off the deep end.

KSE Milestone 2
Note ALL MODULES MUST BE RECOMPILED
make the kernel aware that there are smaller units of scheduling than the
process. (but only allow one thread per process at this time).
This is functionally equivalent to teh previousl -current except
that there is a thread associated with each process.

Sorry john! (your next MFC will be a doosie!)

Reviewed by: peter@freebsd.org, dillon@freebsd.org

X-MFC after: ha ha ha ha

Implement a slightly different window spill/fill algorithm for dealing
with user windows in kernel mode. We split the windows using %otherwin,
but instead of spilling user window directly to the pcb, we attempt to
spill to user space. If this fails because a stack page is not resident
(or the stack is smashed), the fault handler at tl 2 will detect the
situation and resume at tl 1 again where recovery code can spill to the
pcb. Any windows that have been saved to the pcb will be copied out to
the user stack on return from kernel mode.

Add a first stab at 32 bit window handling. This uses much of the same
recovery code as above because the alignment of the stack pointer is used
to detect 32 bit code. Attempting to spill a 32 bit window to a 64 bit
stack, or vice versa, will cause an alignment fault. The recovery code
then changes the window state to vector to a 32 bit spill/fill handler
and retries the faulting instruction.

Add ktr traces in useful places during trap processing.

Adjust comments to reflect new code and add many more.