New option to add an extra header containing the Amavis category.

With this new feature Amavis adds the header "X-Amavis-Category" if $header_category = 1 is found on amavisd.conf.

The X-Amavis-Category contains the category of the email based on ccat_display_names.

It works for me, but I don't know Amavis deeply, so you could improve the mechanism if you like.

This info could be useful to classify emails through external filters (sieve, or IMAP client filters...).

Thank you very much to consider this.