Refactor core dumping code a bit

This makes it possible to use core_write(), core_output(),
and sbuf_drain_core_output(), in Linux coredump code. Moving
them out of imgact_elf.c is necessary because of the weird way
it's being built.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30369