Factor out duplicated kernel dump code into dump_{start,finish}().

dump_start() and dump_finish() are responsible for writing kernel dump
headers, optionally writing the key when encryption is enabled, and
initializing the initial offset into the dump device.

Also remove the unused dump_pad(), and make some functions static now that
they're only called from kern_shutdown.c.

No functional change intended.

Reviewed by: cem, def
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11584