Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC: r253266

Prefix the alias macros for members of struct __mcontext with an underscore
in order to avoid a clash in the net80211 code.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Don't allow user process to set an invalid window state through sigreturn.

Spotted by: tmm

- Expand struct trapframe to 256 bytes, make all fields fixed width and the
same size. Add some fields that previously overlapped with something else
or were missing.
- Make struct regs and struct mcontext (minus floating point) the same as
struct trapframe so converting between them is easy (null).
- Add space for saving floating point state to struct mcontext. This requires
that it be 64 byte aligned.
- Add assertions that none of these structures change size, as they are part
of the ABI.
- Remove some dead code in sendsig().
- Save and restore %gsr in struct trapframe. Remember to restore %fsr.
- Add some comments to exception.S.

1. Remove bootinfo and just pass loader metadata to the kernel.
2. Remove mcontext.mc_sp, it is redundant. Adjust spare space to make
ucontext_t a nice size.
3. Raise pil in the debugger.

Submitted by: tmm (3)

Don't use types that require other headers.

Implement signals.

The author isn't a [UC] Regents. Correct the copyright language.

Flesh out the sparc64 port considerably. This contains:
- mostly complete kernel pmap support, and tested but currently turned
off userland pmap support
- low level assembly language trap, context switching and support code
- fully implemented atomic.h and supporting cpufunc.h
- some support for kernel debugging with ddb
- various header tweaks and filling out of machine dependent structures

Add skeleton machine dependent headers and c files for a port of freebsd
to a new architecture. This is the base of the sparc64 port, but contains
limited machine dependent code, and can be used a base for ports. Included
are:
- standard machine dependent headers, tweaked for a 64 bit, big endian
architecture, including empty versions of all the machine dependent
structures
- a machine independent atomic.h, which can be used until a port has
support for interrupts and the operations really need to be atomic
- stub versions of all the machine dependent functions, which panic
when called and print out the name of the function that needs to
be implemented. functions which are normally in assembly files are
not included, but this should reduce the number of different undefined
references on the first few compiles from hundreds to 5 or 6
Given minimal startup code and console support it should be trivial to
make this compile and run the first few sysinits on almost any architecture.

Requested by: alfred, imp, jhb