MFC r357067: Drop "All Rights Reserved" from all libbe/bectl files

MFC r356876-r356877: add zfs_mount_at

r356876:
libzfs: add zfs_mount_at

This will be used in libbe in place of the internal zmount(); libbe only
wants to be able to mount a dataset at an arbitrary mountpoint without
altering dataset/pool properties. The natural way to do this in a portable
way is by creating a zfs_mount_at() interface that's effectively zfs_mount()
+ a mountpoint parameter. zfs_mount() is now a light wrapper around the new
method.

The interface and implementation have already been accepted into ZFS On
Linux, and the next commit to switch libbe() over to this new interface will
solve the last compatibility issue with ZoL. The next sysutils/openzfs
rebase against ZoL should be able to build libbe/bectl with only minor
adjustments to build glue.

r356877:
libbe: use the new zfs_mount_at()

More background is available in r356876, but this new interface is more
portable across ZFS implementations and cleaner for what libbe is attempting
to achieve anyways.

MFC r355460: libbe: fix build against sysutils/openzfs, part 1

This is the half of the changes required that work as-is with both in-tree
ZFS and the new hotness, sysutils/openzfs. Highlights are less dependency
on header pollution (from somewhere) and using 'mnttab' instead of
'extmnttab'. In the in-tree ZFS, the latter is a #define for the former,
but in the port extmnttab is actually a distinct struct that's a super-set
of mnttab. We really want mnttab here anyways, so just use it.

MFC r349380, r349383, r349455: bectl(8)/libbe(3) fixes

r349380:
libbe(3): mount: the BE dataset is mounted at /

Other parts of libbe(3) were fairly strict on the mountpoint property of the
BE dataset, and be_mount was not much better. It was improved in r347027 to
allow mountpoint=none for depth==0, but this bit was still sensitive to
mountpoint != / and mountpoint != none. Given that other parts of libbe(3)
no longer restrict the mountpoint property here, and the rest of the base
system is generally OK and will assume that a BE is mounted at /, let's do
the same.

r349383:
libbe(3): restructure be_mount, skip canmount check for BE dataset

Further cleanup after r349380; loader and kernel will both ignore canmount
on the root dataset as well, so we should not be so strict about it when
mounting it. be_mount is restructured to make it more clear that depth==0 is
special, and to not try fetching these properties that we won't care about.

r349455:
bectl(8): create non-recursive boot environments

bectl advertises that it has the ability to create recursive and
non-recursive boot environments. This patch implements that functionality
using the be_create_depth API provided by libbe. With this patch, bectl now
works as bectl(8) describes in regards to creating recursive/non-recursive
boot environments.

MFC r347640: libbe(3): Descend into children of datasets w/ mountpoint=none

These datasets will generally be canmount=noauto,mountpoint=none (e.g.
zroot/var) but have children that may need to be mounted. Instead of
skipping that segment for no good reason, descend.

Approved by: re (kib)

MFC r347027-r347028: libbe(3) mountpoint handling fixes

r347027:
libbe(3): Properly mount BEs with mountpoint=none

Instead of pretending to successfully mount them while not actually
mounting anything, we'll now actually mount them *and* claim we mounted them
successfully.

Reported by: ler

r347028:
libbe: set mountpoint=none in be_import

If we're going to set a mountpoint at all, mountpoint=none makes more sense
than mountpoint=/.

MFC bectl(8)/libbe(3): r337663-337664,337667,337697-337699,337800,337805,
337915-337918,337921,337924,337947,337993-337995,338221-338222,338303,
338417,339047,339972,339994,340334,340507-340508,340592-340594,
340635-340636,340722-340723,340974,342466,342849,342903,342911,343335,
343543,343977,343993-343994,344034,344067,344084,345302,345769,
345845-345846,345848,346082

There are simply too many small changes to enumerate; in summary:

bectl(8)/libbe(3) has been introduced from current state in -CURRENT and
added to the stable/11 rescue build. bectl(8) is a tool for managing ZFS
boot environments, largely inspired by beadm. It includes features such as
being able to jail a boot environment or easily mount it for modification.

Relnotes: probably

MFC r342362-r342363: config(8) duplicate option handling

r342362:
config(8): Allow duplicate options to be specified

config(8)'s option handling has been written to allow duplicate options; if
the value changes, then the latest value is used and an informative message
is printed to stderr like so:

/usr/src/sys/amd64/conf/TEST: option "VERBOSE_SYSINIT" redefined from 0 to 1

Currently, this is only a possibility for cpu types, MAXUSERS, and
MACHINE_ARCH. Anything else duplicated in a config file will use the first
value set and error about duplicated options on subsequent appearances,
which is arguably unfriendly since one could specify:

include GENERIC
nooptions VERBOSE_SYSINIT
options VERBOSE_SYSINIT

to redefine the value later anyways.

Reported by: mmacy

r342363:
config(8): Remove all instances of an option when opting out

Quick follow-up to r342362: options can appear multiple times now, so
clean up all of them as needed. For non-OPTIONS options, this has no effect
since they're already de-duplicated.