Proposed fix for #4305

Instead of sending a probe to a random host in 177/8 to attempt to see what the source and destination MACs are (and, in turn, what the Capture module should use when using ```capture_sendto```), this PR makes this process send a random UDP probe to www.metasploit.com but with a TTL of 1, thereby ensuring that it shouldn't make it past the first hop, if at all.  It also makes the probe host and port configurable, cleans up the "secret".

Note that this is a diff against master but has the fix for #4306 from #4311 merged in.  Once #4311 is landed this should update.