livedump: add event handler hooks

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.

Reviewed by: markj, kib (previous version)
Reviewed by: debdrup (manpages)
Reviewed by: Pau Amma <pauamma@gundo.com> (manpages)
MFC after: 3 weeks
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34067