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

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

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

Approved by: re (implicit)

Add section .note.GNU-stack for assembly files used by 386 and amd64.

Add missing END() macros, as per rev 184547 for amd64. The lack of these
is mostly harmless, but it does upset some of valgrind's functionality.

Properly cleanup the stack before jumping to cerror() if rfork(2) fails.

Submitted by: Igor Sysoev <is@rambler-co.ru>
Approved by: re (scottl)

Standardize on our SCM ID style.

Use 'mov' instead of 'lea' for setting the syscall number in %eax as that
is clearer about what we are actually doing.

Requested by: bde

Fix some rather interesting bugs that managed to not turn up in various
testing and real-life applications:
1) If you returned from the thread function, you got a segv instead of
calling _exit() with your return code.
2) clean up some bogus stack management. There was also an underflow
on function return.
3) when making syscalls, the kernel is expecting to have to leave space
for the function's return address. We need to duplicate this. It was
an accident that the rfork syscall actually worked here. :-/

rfork(2) wrapper for simple rfork-style threads. I have lost count of
the number of times I have given this to people and got asked: why isn't
it in libc? It is impossible to do this without assembler glue to reset
the stack for the new child process.

int rfork_thread(flags, stack_addr, start_fnc, start_arg)
int flags; Flags to rfork system call. See rfork(2).
void *stack_addr; Top of stack for thread.
int (*start_fnc)(void *); Address of thread function to call in child.
void *start_arg; Argument to pass to the thread function in child.

This is deliberately not documented or prototyped in includes until the
corresponding alpha version is written.