Refactor some of the MI kernel dump code in preparation for netdump.

• Add clear_dumper() to complement set_dumper().
• Drain netdump's preallocated mbuf pool when clearing the dumper.
• Don't do bounds checking for dumpers with mediasize 0.
• Add dumper callbacks for initialization for writing out headers.

Reviewed by: sbruno
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15252