add ability to set watchdog timeout for a shutdown

This change allows to specify a watchdog(9) timeout for a system
shutdown.  The timeout is activated when the watchdogd daemon is
stopped.  The idea is to a prevent any indefinite hang during late
stages of the shutdown.  The feature is implemented in rc.d/watchdogd,
it builds upon watchdogd -x option.

Note that the shutdown timeout is not actiavted when the watchdogd
service is individually stopped by an operator.  It is also not
activated for the 'shutdown' to the single-user mode.  In those cases it
is assumed that the operator knows what they are doing and they have
means to recover the system should it hang.

Significant subchanges and implementation details:
- the argument to rc.shutdown, completely unused before, is assigned to
  rc_shutdown variable that can be inspected by rc scripts
- init(8) passes "single" or "reboot" as the argument, this is not
  changed
- the argument is not mandatory and if it is not set then rc_shutdown is
  set to "unspecified"
- however, the default jail management scripts and jail configuration
  examples have been updated to pass "jail" to rc.shutdown, just in case
- the new timeout can be set via watchdogd_shutdown_timeout rc option
- for consistency, the regular timeout can now be set via
  watchdogd_timeout rc option
- watchdogd_shutdown_timeout and watchdogd_timeout override timeout
  specifications in watchdogd_flags
- existing configurations, where the new rc options are not set, should
  keep working as before

I am not particularly wed to any of the implementation specifics.
I am open to changing or removing any of them as long as the provided
functionality is the same (or very close) to the proposed one.
For example, I think it can be implemented without using watchdogd -x,
by means of watchdog(1) alone.  In that case there would be a small
window between stopping watchdogd and running watchdog, but I think that
that is acceptable.

Reviewed by:	bcr (man page changes)
MFC after:	5 weeks
Relnotes:	yes
Differential Revision: https://reviews.freebsd.org/D21221

1 files changed, 10 insertions, 1 deletions

diff --git a/sbin/init/init.8 b/sbin/init/init.8
index 976b3307a016..d852c32ef487 100644
--- a/sbin/init/init.8
+++ b/sbin/init/init.8
@@ -31,7 +31,7 @@
 .\"     @(#)init.8	8.3 (Berkeley) 4/18/94
 .\" $FreeBSD$
 .\"
-.Dd August 15, 2018
+.Dd August 6, 2019
 .Dt INIT 8
 .Os
 .Sh NAME
@@ -270,6 +270,15 @@ The timeout can be configured via the
 variable
 .Va kern.init_shutdown_timeout .
 .Pp
+.Nm init
+passes
+.Dq Li single
+as the argument to the shutdown script if return to single-user mode
+is requested.
+Otherwise,
+.Dq Li reboot
+argument is used.
+.Pp
 The role of
 .Nm
 is so critical that if it dies, the system will reboot itself