MFC r301522 (by bz)

Implement a `show panic` command to DDB which will helpfully print the
panic string again if set, in case it scrolled out of the active
window. This avoids having to remember the symbol name.

Also add a show callout <addr> command to DDB in order to inspect
some struct callout fields in case of panics in the callout code.
This may help to see if there was memory corruption or to further
ease debugging problems.

No objection by: bz

MFC r287964:

Kernel part of reroot support - a way to change rootfs without reboot.

Note that the mountlist manipulations are somewhat fragile, and not very
pretty. The reason for this is to avoid changing vfs_mountroot(), which
is (obviously) rather mission-critical, but not very well documented,
and thus hard to test properly. It might be possible to rework it to use
its own simple root mount mechanism instead of vfs_mountroot().

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2698

MFC r274366:

Add missing privilege check when setting the dump device.

Approved by: pjd, secteam (both no objections)
Sponsored by: Multiplay

MFC r288446: Disable suspend during shutdown.

MFC r283115

Properly null-terminate strings in a kernel dump header. A version string
longer than 192 bytes will cause the version field of a dump header to
overflow. strncpy doesn't null terminate it, so savecore will print a
corrupted info file. Using strlcpy fixes the bug.

MFC r281915:
Make vpanic() externally visible.

MFC r281916:
Fix DTrace's panic() action.

MFC r258893, r258956:
Add a new sysctl / loader tunable kern.panic_reboot_wait_time which
defaults to PANIC_REBOOT_WAIT_TIME (a long-existing kernel config
setting). Use this now-variable value in place of the defined constant
to control how long the system waits after a panic before rebooting.

MFC r283115

Properly null-terminate strings in a kernel dump header. A version string
longer than 192 bytes will cause the version field of a dump header to
overflow. strncpy doesn't null terminate it, so savecore will print a
corrupted info file. Using strlcpy fixes the bug.

MFC r281915:
Make vpanic() externally visible.

MFC r281916:
Fix DTrace's panic() action.

MFC r258893, r258956:
Add a new sysctl / loader tunable kern.panic_reboot_wait_time which
defaults to PANIC_REBOOT_WAIT_TIME (a long-existing kernel config
setting). Use this now-variable value in place of the defined constant
to control how long the system waits after a panic before rebooting.