Instead of assuming we have a vnode to dump to, pull out the bits that
describe how to write acoredump into a struct coredump_writer; the ctx in
that is expected to be opaque, but used by write/extend functions. This
should not be a functional change- we change two callers to use the classic
vnode write/extend interface.
This opens us up to other possible targets for coredumps, such as a shmfd.