Some utilities might want to access the EHLO/HELO domain in the
post-data hook (for example, to do additional SPF validations).
This patch implements that support, including sanitizing the EHLO domain
on the environment variable to reduce the risk of problems.
This patch implements two new hooks: alias-resolve and alias-exists.
They are called during the aliases resolution process, to allow for more
complex integration with other systems, such as storing the aliases in a
database.
See the included documentation for more details.
