Use meterpreter dns resolve

This replaces instances of getaddrinfo in post modules in favour of the meterpreter 

Some issues that this threw up:
a) We have a recon folder, but lots of 'recon' takes places in gather/enum_xxx... This really should be standardized?
b) enum_computers and computer_browser_discovery both use the same method but one goes via railgun and the other goes via railgun netapi calls. I have started to put together a library for netapi in https://github.com/rapid7/metasploit-framework/pull/2107. This could be expanded and alternatives for shell access via the command line could be incorporated.
c) I've deprecated the resolve_hostname module. This does have more functionality than the resolve_hosts module so perhaps should keep it? Or remove/deprecate resolve_hosts? I didn't realise it even existed tbh...