Low-level FFI bindings to libmilter, the sendmail mail filter API.
The _milter_ library must be installed on your machine.
For example, on Debian and Ubuntu, milter library and header files are provided
by the packages `libmilter1.0.1` and `libmilter-dev`.
See the [_milter_][1] crate for idiomatic Rust bindings to the milter library.