stand: 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 $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

stand/boot1.efi: use the bootonce dataset as root dataset

Before this change we would only pass the bootonce dataset name
to the environment for the next loader, while actually reading
the next stage loader from the 'bootfs' dataset, not the bootonce
dataset.

Another problem fixed by this change is a boot from a configuration
when bootonce attribute is present, but 'bootfs' property is not set.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40389

stand/boot1.efi: Implement bootonce for ZFS

Implement ZFS bootonce protocol. We pass zfs-bootonce=t to the next boot
stage as a command line argument. Unlike zfsboot -> loader handoff in
the BIOS case, we don't use the OS_BOOTONCE_USED. This would require
modifications to loader.efi which would only server to make it more
complicated. Instead, use the command line parsing interface for the
boot1.efi -> loader.efi to pass in the zfs-bootonce kenv that will be
needed by rc.d/zfsbe to activate the BE if boot progresses that far.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D39412

loader: implement mount/unmount rootfs

We want to keep our root file system open to preserve bcache segment
between file accesses, thus reducing physical disk IO.

Reviewed by: imp, allanjude, kevans (previous version)
Differential Revision: https://reviews.freebsd.org/D30848
MFC after: 1 month

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

Remove left-over status variables

Use simple malloc/free instead of dropping down to the UEFI
BootServices AllocatePool/FreePool calls. They are simpler to use and
result in the same thing happening.

Reviewed by: tsoome@
Differential Revision: https://reviews.freebsd.org/D20540

Don't shadow a global zfsmount variable.

Move sys/boot to stand. Fix all references to new location

Sponsored by: Netflix