sbin: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

loader: zfs should support bootonce an nextboot

bootonce feature is temporary, one time boot, activated by
"bectl activate -t BE", "bectl activate -T BE" will reset the bootonce flag.

By default, the bootonce setting is reset on attempt to boot and the next
boot will use previously active BE.

By setting zfs_bootonce_activate="YES" in rc.conf, the bootonce BE will
be set permanently active.

bootonce dataset name is recorded in boot pool labels, bootenv area.

in case of nextboot, the nextboot_enable boolean variable is recorded in
freebsd:nvstore nvlist, also stored in boot pool label bootenv area.
On boot, the loader will process /boot/nextboot.conf if nextboot_enable
is "YES", and will set nextboot_enable to "NO", preventing /boot/nextboot.conf
processing on next boot.

bootonce and nextboot features are usable in both UEFI and BIOS boot.

To use bootonce/nextboot features, the boot loader needs to be updated on disk;
if loader.efi is stored on ESP, then ESP needs to be updated and
for BIOS boot, stage2 (zfsboot or gptzfsboot) needs to be updated
(gpart or other tools).

At this time, only lua loader is updated.

Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25512

zfsbootcfg: use vfs.root.mountfrom and update nextboot on every vdev

vfs.zfs.boot.primary_pool is only set on BIOS boot, use vfs.root.mountfrom
instead and update all vdevs on pool.

Reviewed by: allanjude
Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25103

zfsbootcfg: a simple tool to set next boot (one time) options for zfsboot

(gpt)zfsboot will read one-time boot directives from a special ZFS pool
area. The area was previously described as "Boot Block Header", but
currently it is know as Pad2, marked as reserved and is zeroed out on
pool creation. The new code interprets data in this area, if any, using
the same format as boot.config. The area is immediately wiped out.
Failure to parse the directives results in a reboot right after the
cleanup. Otherwise the boot sequence proceeds as usual.

zfsbootcfg writes zfsboot arguments specified on its command line to the
Pad2 area of a disk identified by vfs.zfs.boot.primary_pool and
vfs.zfs.boot.primary_vdev kenv variables that are set by loader during
boot. Please see the manual page for more.

Thanks to all who reviewed, contributed and made suggestions! There are
many potential improvements to the feature, please see the review for
details.

Reviewed by: wblock (docs)
Discussed with: jhb, tsoome
MFC after: 3 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D7612