Refactor core dumping code a bit

[trasz]

Summary

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.

[kib]

sys/kern/kern_exec.c

2015

As a follow-up, please somewhat clean this function:

• rename 'p' to e.g. cp
• add local var p for cp->td->td_proc and use it

[trasz]

(Tinderboxed.)

[trasz]

sys/kern/kern_exec.c

2015

https://reviews.freebsd.org/D30397