configurable abort on fatal errors

[elliefm]

When a Cyrus process detects a fatal error, it logs an error message and shuts down with a non-zero exit code. This PR adds a new imapd.conf option fatals_abort, which when enabled, will cause Cyrus to call abort() instead of exiting non-zero. Depending on system configuration, this will usually result in a core dump.

Fastmail:
This will conflict with the "Fastmail ONLY - make assertion failures and fatal errors into coredumps" commit on our capstone branch. To deploy this change at Fastmail, I think we need to do something like:

1. Remove that commit from the capstone branch. New builds will lack the core dump on fatal error behaviour.
2. Label this PR include-in-fastmail. New builds will have the configurable behaviour, but it won't be enabled.
3. Deploy new builds across our environment.
4. Deploy an imapd.conf change to add fatals_abort: yes. We start getting core dumps on fatal errors again.

If we deploy the imapd.conf update before our Cyrus builds include this PR, we might get errors about the unrecognised option. So I think it's better to get the code deployed first, and then turn it on afterwards, though this leaves a window where we won't get core dumps.

We should not land this PR on the master branch until our own deployment process is finished, so that we can use include-in-fastmail to control the timing.

[rsto]

This does not seem to produce a core dump for failed assertions. Is that on purpose?

[elliefm]

This does not seem to produce a core dump for failed assertions. Is that on purpose?

It ought to, assertionfailed() in lib/assert.h has the update to call abort(). Though even if it didn't have that update, it calls fatal() and all the fatals now have the abort anyway.

Where are you seeing it not dump core? Perhaps I should add a test for assertion failures too.

[rsto]

Sorry, my error. I must have overlooked the changes in lib/assert.h during code review.

[brong]

This looks great, thanks for moving it from a local patch to a useful configurable option.