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 $FreeBSD$: one-line .c pattern

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

libsa: Make the nvlist implementation more self-contained

Move declarations into a new nvlist.h rather than putting everything in
libzfs.h. This makes this nvlist code easier to reuse elsewhere. In
particular, the nvlist implementation in sys/contrib/libnv does not
provide XDR encoding, but this is needed when reading from or writing to
ZFS pools.

Also:
- Remove references to boolean_t. It has to be a 32-bit int here, so
just reference the underlying type.
- Add includes needed when compiling the nvlist code outside of stand/.

No functional change intended.

Reviewed by: tsoome
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35255

libsa: Fix a bug in nvlist creation

When adding an entry to an nvlist, the data buffer might need to be
resized. When this happens, the XDR encoder's notion of the buffer size
also needs to be updated, otherwise the operation may erroneously fail.

Reviewed by: tsoome, imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35254

libsa: xdrproc_t should return bool

Since our xdr translation function are returning bool, so should
xdrproc_t.

Issue reported by gcc 10 build.

loader: xdr_array is missing count

The integer arrays are encoded in nvlist as counted array <count, i0, i1...>,
loader xdr_array() is missing the count. This will affect the pool import when
there are hole devices in pool.

Also fix the new data add and print functions.

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

loader: create single zfs nextboot implementation

We should have nextboot feature implemented in libsa zfs code.
To get there, I have created zfs_nextboot() implementation based on
two sources, our current simple textual string based approach with added
structured boot label PAD structure from OpenZFS.

Secondly, all nvlist details are moved to separate source file and
restructured a bit. This is done to provide base support to add nvlist
add/update feature in followup updates.

And finally, the zfsboot/gptzfsboot disk access functions are swapped to use
libi386 and libsa.

Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25324