[build2] Changes to build emails

[Boris Kolpackov]

If you are the author of a package on cppget.org, then you may be
receiving its build results via email. Whether such emails are sent
as well as for which results is controlled by the build-email,
build-warning-email, and build-error-email package manifest values[1].

Up until now, if neither of these values were specified, then we
would sent build emails for all results to the address specified
with package-email, or, if that it not specified, with the email
manifest value.

In retrospect, this semantics was too implicit and did not have a
sufficiently explicit opt-in (which is always a good idea with email).
As a result, we've changed this and will now send build emails only
if one of the build*-email manifest values is specified explicitly.

Note that while the manifest documentation that reflects this change
will only be updated with the upcoming release, we've already deployed
the change to cppget.org and you may have noticed that you've stopped
receiving build emails for packages that did not have build*-email.
If you are not interested in receiving build emails for such packages,
then there is nothing to do.

If, however, you would like to continuing receiving such emails, then
you will need to release and publish revisions of such packages that
explicitly specifies a suitable build*-email value. For instructions
on how to release and publish a revision, see Step #3 in Version and
Release Management[2]

Let me know if you have any questions or concerns and thank you for
your packaging efforts.

[1] https://build2.org/bpkg/doc/build2-package-manager-manual.xhtml#manifest-package-build-email
[2] https://github.com/build2-packaging/README#version-and-release-management