Revise email_addresses and rename it to directory_email_addresses

Revise email_addresses and rename it to directory_email_addresses

This also adds a new feature:  email addresses can be registered only for
certain subdirectories belonging to a package if desired, by listing them
in an "email" file at the corresponding location in the configuration tree.
This uses the same code as is used to locate authorized keyrings.

The use of pattern matching to extract email addresses is a precaution,
although observant readers may notice that the patterns are very lax.
Perl taint mode checks do not require this, since the email addresses will
be written to a socket rather than passed as command arguments, and hash
keys, used here for efficient de-duplication, do not carry taintedness.