Support SRVHOST option for exploits/windows/browser/msvidctl_mpeg2.rb

[wchen-r7]

For #4133

To verify this, this is what I did:

• Put a print_debug(host.inspect) at line 198
• Run the module
• In another terminal, curl -v the exploit's uri
• curl -v will tell you where you should redirect to. Just follow that and curl again.
• The print_debug should trigger and print either SRVHOST or Rex::Socket.source_address depending on how you set it. If you change SRVHOST to something else other than 0.0.0.0, then the module will use that. Otherwise it will fall back to Rex::Socket.source_address.
• When you're done testing, make sure you undo the code change before landing.

[jhart-r7]

Configured eth0:0 as 10.0.1.200 (eth0 is 10.0.1.18).

Set SRVHOST to 0.0.0.0 (the default). Before fix: http://10.0.1.18:8080. After fix: http://10.0.1.18:8080

Set SRVHOST to 10.0.1.200. Before fix: http://10.0.1.18:8080 (incorrect). After fix: http://10.0.1.200:8080 (correct)