Extra support for some archive types

[Ralph Seichter]

* korsar:

> I mean 7zip supports 35 archive type, any of them may be used to hide
> ransomware in the .rar or .zip file and opened by end user. So the
> mail system is only half protected.

Amavis' default settings are not meant to offer complete protection,
just a reasonable basis to start from. Adding an unpacker for archive
type X or banning a type Y can be achieved by changing amavisd.conf
alone.

> How I can make filetype-to-decoder mappings in amavisd.conf?

Search https://gitlab.com/amavis/amavis/blob/master/amavisd.conf for the
@decoders array if you need examples. The description used to read:

"A list of pairs or n-tuples: [short-type, code_ref, optional-args...].
Maps short types to a decoding routine, the first match wins. Arguments
beyond the first two can be program path string (or a listref of paths
to be searched) or a reference to a variable containing such a path,
which allows for lazy evaluation, making possible to assign values to
legacy configuration variables even after the assignment to @decoders."

-Ralph