livedump: add event handler hooks

[mhorne]

Summary

Add three hooks to the livedump process: before, after, and for each
block of dumped data. This allows, for example, quiescing the system
before the dump begins or protecting data of interest to ensure its
consistency in the final output.

[pauamma_gundo.com]

Change language grammar and spelling LGTM. I'll let others check consistency with source code.

share/man/man9/EVENTHANDLER.9

328

Since you're touching this section

[debdrup]

mdoc looks good, you can fix the nit Pau Amma pointed out

[kib]

sys/kern/kern_vnodedumper.c

59

How could this declaration used? It is only available in this .c file, so all users must be in this .c file as well. Then why not call them directly, instead of using the machinery of eventhandlers?

[mhorne]

Move handler declarations to kerneldump.h.

[markj]

sys/kern/kern_vnodedumper.c

107

Save the minidumpsys() error in a separate variable? Why is it useful to allow the _finish handlers to return an error?

[pauamma_gundo.com]

LGTM now as well.

[mhorne]

Remove the error arg from the livedumper_finish event.

[pauamma_gundo.com]

English LGTM. Committable if others OK content.

[markj]

share/man/man9/EVENTHANDLER.9

324

Might want to s/minidump/kernel dump/ per the discussion in the vnode dumper review.