dtrace: Add the 'oformat' libdtrace option

This option can be used to specify a format to use in DTrace output.
The following formats are supported:
- json
- xml
- html
- none (default DTrace output)

This is implemented using libxo and integrated into libdtrace. Client
code only works with the following API:

- dtrace_oformat_setup(dtrace_hdl_t *) -- to be called when output is starting.
- dtrace_oformat_teardown(dtrace_hdl_t *) -- to be called when output is finished
- dtrace_oformat(dtrace_hdl_t *) -- check if oformat is enabled.
- dtrace_set_outfp(FILE *) -- sets the output file for oformat.
- Ensure that oformat is correctly checked in the drop handler and record
processing callbacks.

This commit also adds tests which check if the generated output is
valid (JSON, XML) and extends the dtrace(1) describing the structured output.

Reviewed by: markj
Discussed with: phil
MFC after: 2 months
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41745

Merge OpenZFS support in to HEAD.

The primary benefit is maintaining a completely shared
code base with the community allowing FreeBSD to receive
new features sooner and with less effort.

I would advise against doing 'zpool upgrade'
or creating indispensable pools using new
features until this change has had a month+
to soak.

Work on merging FreeBSD support in to what was
at the time "ZFS on Linux" began in August 2018.
I first publicly proposed transitioning FreeBSD
to (new) OpenZFS on December 18th, 2018. FreeBSD
support in OpenZFS was finally completed in December
2019. A CFT for downstreaming OpenZFS support in
to FreeBSD was first issued on July 8th. All issues
that were reported have been addressed or, for
a couple of less critical matters there are
pull requests in progress with OpenZFS. iXsystems
has tested and dogfooded extensively internally.
The TrueNAS 12 release is based on OpenZFS with
some additional features that have not yet made
it upstream.

Improvements include:
project quotas, encrypted datasets,
allocation classes, vectorized raidz,
vectorized checksums, various command line
improvements, zstd compression.

Thanks to those who have helped along the way:
Ryan Moeller, Allan Jude, Zack Welch, and many
others.

Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D25872

Avoid adding an extra "0x" prefix before pointer formats.

MFC after: 1 week

Fix some incorrect endianness checks.

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division