kern: abstract away the vnode coredumper to allow pluggable dumpers

The default and only stock coredumper will continue to be the
traditional vnode dumper, which will dump to a vnode and issue a devctl
notification. With this change, one can write a kmod that injects custom
handling of user coredumps that offers richer behavior, particularly in
case one wants to add more metadata than we can tap out via devd.

The main motivation here is to pave the way for my usercore daemon to be
able to reroute coredumps before they ever touch the disk. In some
cases they may be discarded and we can avoid the overhead of writing
anything, in others they allow us to capture coredumps that would be
written into an area that's transient in nature (e.g., kyua test work
directories) without having to do more tricks to keep those alive. My
WIP kmod writes the coredump into a shmfd instead of a vnode, then
installs that into ucored(8) with every read(2) of /dev/ucore. This
also allows me to capture more metadata reliably before the process and
jail disappear.

Reviewed by: kib (earlier version), markj
Differential Revision: https://reviews.freebsd.org/D51338