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)

sysctl(9) cleanup checkpoint: amd64 GENERIC builds cleanly.

Commit the kernel changes.

Rename debug.ddb.capture.bytes sysctl to debug.ddb.capture.bufoff in
order to match the internal variable name.

Add a new sysctl debug.ddb.capture.inprogress to export the inprogress
variable.

MFC after: 3 days

Increase maximum DDB capture buffer size to 5MB.

PR: 119993
MFC after: 2 months
Suggested by: Scot Hetzel <swhetzel at gmail dot com>

Allow DDB_CAPTURE_DEFAULTBUFSIZE and DDB_CAPTURE_MAXBUFSIZE to be
overridden at compile-time using kernel options of the same names.

Rather than doing a compile-time CTASSERT of buffer sizes being
even multiples of block sizes, just adjust them at boottime, as
the failure mode is more user-friendly.

MFC after: 2 months
PR: 119993
Suggested by: Scot Hetzel <swhetzel at gmail dot com>

Rename DB_ constants in db_capture.c to DDB_ so that when they are
exposed as kernel compile options, they have more meaningful names.

PR: 119993
MFC after: 2 months
Suggested by: Scot Hetzel <swhetzel at gmail dot com>

Add textdump(4) facility, which provides an alternative form of kernel
dump using mechanically generated/extracted debugging output rather than
a simple memory dump. Current sources of debugging output are:

- DDB output capture buffer, if there is captured output to save
- Kernel message buffer
- Kernel configuration, if included in kernel
- Kernel version string
- Panic message

Textdumps are stored in swap/dump partitions as with regular dumps, but
are laid out as ustar files in order to allow multiple parts to be stored
as a stream of sequentially written blocks. Blocks are written out in
reverse order, as the size of a textdump isn't known a priori. As with
regular dumps, they will be extracted using savecore(8).

One new DDB(4) command is added, "textdump", which accepts "set",
"unset", and "status" arguments. By default, normal kernel dumps are
generated unless "textdump set" is run in order to schedule a textdump.
It can be canceled using "textdump unset" to restore generation of a
normal kernel dump.

Several sysctls exist to configure aspects of textdumps;
debug.ddb.textdump.pending can be set to check whether a textdump is
pending, or set/unset in order to control whether the next kernel dump
will be a textdump from userspace.

While textdumps don't have to be generated as a result of a DDB script
run automatically as part of a kernel panic, this is a particular useful
way to use them, as instead of generating a complete memory dump, a
simple transcript of an automated DDB session can be captured using the
DDB output capture and textdump facilities. This can be used to
generate quite brief kernel bug reports rich in debugging information
but not dependent on kernel symbol tables or precisely synchronized
source code. Most textdumps I generate are less than 100k including
the full message buffer. Using textdumps with an interactive debugging
session is also useful, with capture being enabled/disabled in order to
record some but not all of the DDB session.

MFC after: 3 months

Add a new DDB(4) facility, output capture. Input and output from DDB may be
captured to a memory buffer for later inspection using sysctl(8), or in the
future, to a textdump.

A new DDB command, "capture", is added, which accepts arguments "on", "off",
"reset", and "status".

A new DDB sysctl tree, debug.ddb.capture, is added, which can be used to
resize the capture buffer and extract buffer contents.

MFC after: 3 months