mount root hooks for non-disk-device devices

[Simon Burge <simonb%NetBSD.org@localhost>]

Hi folks,

As part of my boot-from-ZFS work, I've generalised how "non-disk"
disk-type storage devices interface with the mountroot mechanism.
This change adds "mount root hooks", which allows a device driver
to advertise that it can contain a virtual storage device.  The new
interface is:

    mountroot_rootspec_hook_establish()
        Called by a driver to register a callback to check if the passed
        boot device string is matched by a driver and a callback to
        print a list of any boot devices provided by the driver.

	The match function also passes a string prefix (eg "wedge") to
	register the hook.

    mountroot_rootspec_hook_disestablish()
	Remove callbacks previously registered.

    domountroot_rootspec_hooks()
        Called during the mount root process.  This checks if registered
        prefixes match any listed hooks and calls them.  The prefix is
        can be separated from the boot specifier with either a ":" or an
        "="; for example "wedge:foo", "NAME=bar".

    domountroot_rootspec_print()
	Called when the boot device prompt lists the available boot
	devices.

These changes remove most wedge-specific code from sys/kern.  One change
in behaviour is that both ":" and "=" can be used to separate a prefix
from the target.  For example "NAME=foo" and "NAME:foo" will both work.

This has been tested on amd64 with FFS and ZFS for a while, but has only
had limited testing on other architectures.

The diffstats output is below, and the diff is available at
https://www.netbsd.org/~simonb/mountroot-hooks-20240501.diff

 sys/dev/dkwedge/dk.c |   34 ++++++++++++++++++-
 sys/kern/kern_hook.c |   87 ++++++++++++++++++++++++++++++++++++++++++++++++++
 sys/kern/kern_subr.c |   31 ++---------------
 sys/sys/systm.h      |   12 ++++++
 4 files changed, 135 insertions(+), 29 deletions(-)