SROP mitigation. sendsig() stores a (per-process ^ &sigcontext) cookie
inside the sigcontext. sigreturn(2) checks syscall entry was from the
exact PC addr in the (per-process ASLR) sigtramp, verifies the cookie,
and clears it to prevent sigcontext reuse.
not yet tested on landisk, sparc, *88k, socppc.
ok kettenis

Do not expose the trap T_* values to userland, since they invade the
namespace a bit much. These are the names for the value encoded in
core.c_ucode or siginfo.si_trapno, but userland never looks directly
and only cares about the abstract signal code.
ok guenther

Determine whether we're currently on the alternative signal stack
dynamically, by comparing the stack pointer against the altstack
base and size, so that you get the correct answer if you longjmp
out of the signal handler, as tested by regress/sys/kern/stackjmp/.
Also, fix alt stack handling on vax, where it was completely broken.

Testing and corrections by miod@, krw@, tobiasu@, pirofti@

merge various differences between hppa and hppa64
ok jsing

Normalize sentinel. Use _MACHINE_*_H_ and _<ARCH>_*_H_ properly and consitently.

Discussed and okay drahn@. Okay deraadt@.

Expose struct sigcontext, struct sigstat and struct sigaltstack for XPG4.2

convert _FOO_SOURCE -> __FOO_VISIBLE in machine. OK deraadt@

make it look more like the other archs

make 'em all unsigned

save the fpu state for the signal handler. this is essential
as the fpu regs can be used by the gcc even for non-fpu
means and data copying.
rearrange the sigcontext a bit to include only the general
registers that are needed restoring upon return and thus
less waste of space and make up some space for the fpu regs.

ALL software that used the signal context on the stack
needs at least rebuilding now. a new snap has bin built as well.

implement signal delivery; based on fredette@'s work

branches: 1.1.8; 1.1.18;
some includes