dtrace: Add the 'oformat' libdtrace option

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