Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

MFC 256941: make sure the frame is indeed in the kernel memory.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Add a comment explaining why stack_capture is empty for EABI and clang.

While here add a comment pointing out that, while r11 is not the frame
pointer on EABI as there is no frame pointer, it's value is unused so is
safe.

Don't use the old stack-walking code with
EABI ARM kernels or clang-compiled ARM kernels.

This fixes a crash seen in clang-compiled ARM
kernels that include WITNESS.

This code could be easily modified to walk the stack
for current clang-generated code (including EABI)
but Andrew Turner has raised concerns that the
stack frame currently emitted by clang isn't actually
required by EABI so such a change might cause problems
down the road.

In case anyone wants to experiment, the change
to support current clang-compiled kernels
involves simply setting FR_RFP=0 and FR_SCP=1.

Break out stack(9) from ddb(4):

- Introduce per-architecture stack_machdep.c to hold stack_save(9).
- Introduce per-architecture machine/stack.h to capture any common
definitions required between db_trace.c and stack_machdep.c.
- Add new kernel option "options STACK"; we will build in stack(9) if it is
defined, or also if "options DDB" is defined to provide compatibility
with existing users of stack(9).

Add new stack_save_td(9) function, which allows the capture of a stacktrace
of another thread rather than the current thread, which the existing
stack_save(9) was limited to. It requires that the thread be neither
swapped out nor running, which is the responsibility of the consumer to
enforce.

Update stack(9) man page.

Build tested: amd64, arm, i386, ia64, powerpc, sparc64, sun4v
Runtime tested: amd64 (rwatson), arm (cognet), i386 (rwatson)