[arch-dev-public] heirloom-mailx ==> s-nail

FYI, I'm replacing heirloom-mailx from [core] which has been unmaintained upstream for more than two years by s-nail, a recent fork which is still vastly similar to heirloom-mailx but brings many small features, clean ups, and general improvements. It is currently in [testing]. Cheers. -- Gaetan