mtools v3.1

Changes

• Add optional -s ADDR argument for SSM to mreceive, issue #3
• Add support for quiet (-q) mode operation
• Add credit to original authors in README and man pages
• Refactor the option parsers to use getopt(3)
• Major refactoring and coding style cleanup, splitting the code base up
  in more files, renaming internal APIs, all to be able to use RFC3678
  style APIs for source filtering, issue #3

Fixes

• Boundary checking, max number of interfaces, internal buffer lengths
• Fixes for building on non-Linux systems, tested on FreeBSD 11.0