SMCRoute v2.2.0

Changes

• Support for dropping root privileges after opening the multicast
  routing socket and creating the PID file
• Support for Source Specific Multicast group subscription (only IPv4)
• Support for systemd, service file included and installed by default

Fixes

• Remove GNUisms to be able to build and run on Alpine Linux (musl libc)
• Add OpenBSD queue.h for systems that do not have any *BSD sys/queue.h
• Coding style cleanup and minor refactor