Log Message:
0.7:
- Document the 'target' configuration directive.
- Merging OS-specific networking code to reduce LOCs and the
  sea of #ifdefs.
- Added 50ms timeout to pcap_open_live() to reduce CPU usage
  on network-heavy hosts. Pcap recommends we not use zero.

0.6:
- Cleanup: Don't null-check before free
- Cleanup: Consolidate flag-check logic
- Accept single-knock sequences
- Introduce a 'target' configuration directive, enabling
  knockd to react to connect attempts to a target host.
  Useful in cases where knockd is on a router and you want
  to send a target a wakeup packet.