kern: move the vnode user coredumper out into its own file

This more cleanly contains the bits that are specifically relevant to
dumping coredumps out to a vnode, which will make future changes a bit
easier to review. This also makes the scope of the relevant sysctls
easier to reason about in the process, as they're not visible outside of
the vnode dumper file -- this will mostly become relevant when we allow
pluggable dumpers.

While we're here, move all of the coredump-related stuff out into its
own kern_ucoredump.c and sys/ucoredump.h. We have enough that it's
useful to separate it out and de-clutter kern_sig.c and sys/exec.h a
bit.

Reviewed by: kib, markj (both earlier version)
Differential Revision: https://reviews.freebsd.org/D51349