r14898: This change is an attempt to improve the quality of the information that

r14898: This change is an attempt to improve the quality of the information that
is produced when a process exits abnormally.

First, we coalesce the core dumping code so that we greatly improve our
odds of being able to produce a core file, even in the case of a memory
fault. I've removed duplicates of dump_core() and split it in two to
reduce the amount of work needed to actually do the dump.

Second, we refactor the exit_server code path to always log an explanation
and a stack trace. My goal is to always produce enough log information
for us to be able to explain any server exit, though there is a risk
that this could produce too much log information on a flaky network.

Finally, smbcontrol has gained a smbd fault injection operation to test
the changes above. This is only enabled for developer builds.
(This used to be commit 56bc02d64498eb3faf89f0c5452b9299daea8e95)