Allow allowlisting allowed forward commands

[DemiMarie]

If smtp.conf was allowed to contain a list of commands allowed in ~/.forward files, most of the security problems associated with ~/.forward files could be avoided. Since the allowed script can still do whatever it wants, this is not a loss in functionality from the user’s perspective.

Such an option should probably be disabled by default, at least at first.

[DemiMarie]

I think a reasonable approach would be to use one or more regular expressions. For instance, ^~/.local/mda/[A-Za-z][A-Za-z0-9._-]*$ would only allow executing MDAs in a subset of paths under ~/.local/mda/. This will require various changes elsewhere first.