VS 2012 version of ext_server_sniffer with bug fix and code tidy

This commit re-adds `ext_server_sniffer` to the solution and provides a few other things:

1. 2 new configurations: `r7_release` and `r7_debug`. These builds include the `ext_server_sniffer` project in the build. Inside VS, R7 employees should use these configurations while open-source contributors should use the normal `release` and `debug` configurations. This means that everyone can continue to use the same solution for development.
1. Command line build updates which check to see if the appropriate SDK (pcap) is present on disk. If it finds it, it'll build the `ext_server_sniffer`, otherwise it won't. This means that the same make command can be used by both Rapid7 employees and non-employees who don't have the pcap library and it'll **Just Work (tm)** without saying that it can't find header files.
1. Removal of more warnings in the code.
1. A fix for the `ext_server_sniffer` build which prevents it from crashing on load.