Ncat's proxy connect option should use Nsock proxychains

[dmiller-nmap]

Ncat currently uses custom proxy connection code in ncat_connect.c. We should replace this with Nsock's proxychains support to keep all such code in one place. Some new features must be added, but we will get more features for Ncat in the process.

Things missing from Nsock that Ncat implements:

1. Remote name resolution or connect-by-name for proxy types that allow this (HTTP and SOCKS 4a/5 allow these, but Nsock doesn't have a good way currently to handle it).

New features that Ncat will gain by this:

1. Ability to chain more than one proxy.
2. More transparent SSL connection capability (was entirely broken, and the fix to #2065 uses nsock_reconnect_ssl() with an IOD converted from a plain socket with nsock_iod_new2(), which the documentation says isn't really supported).

[nnposter]

AFAICT, proxy authentication and SOCKS5 support are also missing from Nsock but present in Ncat.

[digininja]

I've just hit the bug in #2065 and after some searching landed here. Any plans to implement this?