Reject disallowed outgoing packets instead of dropping them

[pobrn]

This lets applications know that communication will not work and thus they can react appropriately much faster instead of running into long timeouts.

Fixes #78

[radl97]

With TCP, it is often recommended to use TCP rejection packet instead of ICMP. (Same with UDP maybe?)

[pobrn]

We could indeed reply with TCP RST but I think this is plenty good. The kernel will recognize that the communicate won't go through and signal it to the application, which is all we need.