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

Add support for a configurable output channel to witness(4).

This is useful in environments where system configuration is performed by
automated interaction with the system console, since unexpected witness
output makes such automation difficult. With this change, the new
debug.witness.output_channel sysctl allows one to specify that witness
output is to be printed to the kernel log (using log(9)) rather than the
console.

Reviewed by: cem, jhb
MFC after: 2 weeks
Relnotes: yes
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4183

Constify stack argument for functions that don't modify it.

Reviewed by: ed, kib, jhb

Add some FEATURE macros for various features (AUDIT/CAM/IPC/KTR/MAC/NFS/NTP/
PMC/SYSV/...).

No FreeBSD version bump, the userland application to query the features will
be committed last and can serve as an indication of the availablility if
needed.

Sponsored by: Google Summer of Code 2010
Submitted by: kibab
Reviewed by: arch@ (parts by rwatson, trasz, jhb)
X-MFC after: to be determined in last commit with code from this project

kdb_backtrace: use stack_print_ddb instead of stack_print

This is a followup to r212964.
stack_print call chain obtains linker sx lock and thus potentially may
lead to a deadlock depending on a kind of a panic.
stack_print_ddb doesn't acquire any locks and it doesn't use any
facilities of ddb backend.
Using stack_print_ddb outside of DDB ifdef required taking a number of
helper functions from under it as well.

It is a good idea to rename linker_ddb_* and stack_*_ddb functions to
have 'unlocked' component in their name instead of 'ddb', because those
functions do not use any DDB services, but instead they provide unlocked
access to linker symbol information. The latter was previously needed
only for DDB, hence the 'ddb' name component.

Alternative is to ditch unlocked versions altogether after implementing
proper panic handling:
1. stop other cpus upon a panic
2. make all non-spinlock lock operations (mutex, sx, rwlock) be a no-op
when panicstr != NULL

Suggested by: mdf
Discussed with: attilio
MFC after: 2 weeks

Add stack_print_short() and stack_print_short_ddb() interfaces to
stack(9), which generate a more compact rendition of a stack trace
via the kernel's printf.

MFC after: 1 week

Make it possible to compile kernel with KTR but without DDB.

Ktr(9) stores format string and arguments in the event circular buffer,
not the string formatted at the time of CTRX() call. Stack_ktr(9) uses
an on-stack buffer for the symbol name, that is supplied as an argument
to ktr. As result, stack_ktr() traces show garbage or cause page faults.

Fix stack_ktr() by using pointer to module symbol table that is supposed
to have a longer lifetime.

Tested by: pho
MFC after: 1 week

Staticize M_STACK.

Approved by: rwatson (mentor)
MFC after: 1 month

When a symbol name can't be resolved, return "??" as the name, rather
than "Unknown func", in order to avoid putting spaces in what ideally
is a string separated by white space.

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)

Modify stack(9) stack_print() and stack_sbuf_print() routines to use new
linker interfaces for looking up function names and offsets from
instruction pointers. Create two variants of each call: one that is
"DDB-safe" and avoids locking in the linker, and one that is safe for
use in live kernels, by virtue of observing locking, and in particular
safe when kernel modules are being loaded and unloaded simultaneous to
their use. This will allow them to be used outside of debugging
contexts.

Modify two of three current stack(9) consumers to use the DDB-safe
interfaces, as they run in low-level debugging contexts, such as inside
lockmgr(9) and the kernel memory allocator.

Update man page.

Correct typos

MFC after: 2 weeks

Add 'depth' argument to CTRSTACK() macro, which allows to reduce number
of ktr slots used. If 'depth' is equal to 0, the whole stack will be
logged, just like before.

- Add support for saving stack traces and displaying them via printf(9)
and KTR.

Contributed by: Antoine Brodin <antoine.brodin@laposte.net>
Concept code from: Neal Fachan <neal@isilon.com>