Fix name resolution for socks4 proxy.

[landgraf]

Previously client didn't try to resolve hostname prior to sending
request to proxy. It violates SOCKS4a specification which says
DSTIP should be set to .X if client cannot resolve the destination
host and may be problematic in some environments.
This behaviour can be overriden with --nodns option (to force client to
use socks4a extention);

[nnposter]

The change, if I understand it correctly, would substantially break backwards compatibility. Namely, I can currently connect like this:

ncat --proxy myproxy.org --proxy-type socks4 mytarget.org 80

which performs remote DNS lookup of mytarget.org by the proxy. With your change, I am supposed to do this instead:

ncat --nodns --proxy myproxy.org --proxy-type socks4 mytarget.org 80

which will fail, because myproxy.org is not an IP address. So I would be forced to perform the proxy lookup myself prior to invoking ncat.

I believe that a better approach could be to decouple hostname resolution of proxy destinations from all the other resolutions, which need to be inherently done locally.

Could you please take a look at #1439 and let me know if it would meet your needs?

[nnposter]

Ability to control DNS resolution implemented in r37586. Thank you for raising up the issue.