Add 4Kn kernel dump support

(And 4Kn minidump support, but only for amd64.)

Make sure all I/O to the dump device is of the native sector size. To
that end, we keep a native sector sized buffer associated with dump
devices (di->blockbuf) and use it to pad smaller objects as needed (e.g.
kerneldumpheader).

Add dump_write_pad() as a convenience API to dump smaller objects with
zero padding. (Rather than pull in NPM leftpad, we wrote our own.)

Savecore(1) has been updated to deal with these dumps. The format for
512-byte sector dumps should remain backwards compatible.

Minidumps for other architectures are left as an exercise for the
reader.

PR: 194279
Submitted by: ambrisko@
Reviewed by: cem (earlier version), rpokala
Tested by: rpokala (4Kn/512 except 512 fulldump), cem (512 fulldump)
Relnotes: yes
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5848