Create a temporary multiprocessing pool

Add check to preflight for exec on tmp

Allow files in /home/user-data/mail/mailboxes

(re-fix) mail sent from an address on a subdomain of a domain hosted by the box (a non-zone domain) would never be DKIM-signed because only zones were included in the openDKIM configuration, mistakenly

This was originally fixed in 143bbf37 (February 16, 2015). Then I broke it in 7a93d219 (November 2015) while doing some refactoring ahead of v0.15.

Reduce percentages for required free memory checks

When previous panel was login, move to system_status

Reduce number of processes in the pool to 5

Fix small typo in comments

the logic in 4ed23f44 for taking backups more often was partly backward

This reverts commit b32cb622.

Fixes #627. Fixes #653. Closes #714.

Fix a typo in Readme

`matchs` → `matches`

Provisioning could crash if, e.g., the DNS we see is different from the DNS Let's Encrypt sees.

see #695, probably fixes it

Added a warning to the installation / setup script

See pull request #638 and issue #635 for more information.

before the user agrees to Let's Encrypt's ToS the admin could get a nightly email with weird interactive text

Made a mistake refactoring the headless variable earlier.

fixes #696

https://discourse.mailinabox.email/t/status-check-emails-empty-after-upgrading-to-v0-16/1082/3

A user on that thread suggests an alternate solution, adding `PYTHONIOENCODING=utf-8` to `/etc/environment`. Python docs say that affects stdin/out/err. But we also use these environment variables elsewhere to ensure that config files we read/write are opened with UTF8 too. Maybe all that can be simplified too.