This commit simplifies the help-string definitions and makes them more
consistent with the upstream configuration of the dovecot/core project.

Drop the inline definition of the TEST_WITH macro in favor of using a
verbatim copy of the file that contains the macro from the dovecot/core
project. This allows easier tracking of changes to the macro that happen
upstream.

Instead use the detected ones.

It adds the ability to manually invoke Sieve filtering in IMAP.

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.