Accept Guard v1.1

Changes

• Add support for UDP services by wrapping recvfrom()
• Add support for IPv6
• Scope ACL environment variable with ACCEPT_GUARD_ prefix to avoid
  clashing with other uses of ACL on the system. Incompatible change!
• Simplify build system slightly
• Add basic test suite to verify accept guard, based on unshare
• Replace unsafe strncpy() with safer version that NUL terminates.
  In a world of systemd named interfaces we are always at IFNAMSIZ

Fixes

• Check return value from getenv(), may be NULL