In effect, this adds PIE and RELRO to binaries.

It allows sending feedback report (RFC 5965) messages.

The actual version numbers live only on the release-branches from now on.

This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.

This was quite fundamentally broken, not only due to previous change.