- Extend the KDB interface to add a per-debugger callback to print a
  backtrace for an arbitrary thread (rather than the calling thread).
  A kdb_backtrace_thread() wrapper function uses the configured debugger
  if possible, otherwise it falls back to using stack(9) if that is
  available.
- Replace a direct call to db_trace_thread() in propagate_priority()
  with a call to kdb_backtrace_thread() instead.

Attempt to make break-to-debugger and alternative break-to-debugger more
accessible:

(1) Always compile in support for breaking into the debugger if options
    KDB is present in the kernel.

(2) Disable both by default, but allow them to be enabled via tunables
    and sysctls debug.kdb.break_to_debugger and
    debug.kdb.alt_break_to_debugger.

(3) options BREAK_TO_DEBUGGER and options ALT_BREAK_TO_DEBUGGER continue
    to behave as before -- only now instead of compiling in
    break-to-debugger support, they change the default values of the
    above sysctls to enable those features by default.  Current kernel
    configurations should, therefore, continue to behave as expected.

(4) Migrate alternative break-to-debugger state machine logic out of
    individual device drivers into centralised KDB code.  This has a
    number of upsides, but also one downside: it's now tricky to release
    sio spin locks when entering the debugger, so we don't.  However,
    similar logic does not exist in other device drivers, including uart.

(5) dcons requires some special handling; unlike other console types, it
    allows overriding KDB's own debugger selection, so we need a new
    interface to KDB to allow that to work.

GENERIC kernels will now support break-to-debugger as long as appropriate
boot/run-time options are set, which should improve the debuggability of
kernels significantly.

MFC: r225214 (partial)

Follow up to r225203 refining break-to-debugger run-time configuration
improvements:

(1) Implement new model in previously missed at91 UART driver
(2) Move BREAK_TO_DEBUGGER and ALT_BREAK_TO_DEBUGGER from opt_comconsole.h
    to opt_kdb.h (spotted by np)
(3) Garbage collect now-unused opt_comconsole.h

Typo.

Trim an additional unnecessary <linker_set.h> include.

- Add the possibility to reuse the already last used timeout when patting
  the watchdog, via the watchdog(9) interface.
- Add the possibility to pat the watchdogs installed via the watchdog(9)
  interface from the kernel.
- Avoid to pass WD_ACTIVE down in the watchdog handlers. All the control
  bit processing should over to the upper layer functions and not passed
  down to the handlers at all.
- Add the watchdogs patting during the (shutdown time) disk syncing and
  disk dumping.

Sponsored by:	Sandvine Incorporated

Extend the DDB command watchdog with the ability to specify a timeout
value.

cannot be MFC'd as it requires a non-MFCable change (adding drains to
sbuf)].

Modify kdb_trap() so that it re-calls the dbbe_trap function as long as
the debugger back-end has changed.  This means that switching from ddb
to gdb no longer requires a "step" which can be dangerous on an
already-crashed kernel.

Also add a capability to get from the gdb back-end back to ddb, by
typing ^C in the console window.

While here, simplify kdb_sysctl_available() by using
sbuf_new_for_sysctl(), and use strlcpy() instead of strncpy() since the
strlcpy semantic is desired.

replace a couple of instances of the same definitions with one in
an include file

Use _SIG_VALID().