Add SOCKS5 proxy support

[nipos]

It would be nice to have support for using a SOCKS5 proxy with Haiku-native applications, most importantly in WebPositive which already supports HTTPS proxys at least. It would be best to implement that in system settings and make it available to every application, as SOCKS5 can proxy every type of connection, not just web content.

Qt already supports SOCKS5 globally, maybe one can look at how it's done there.

SOCKS5 is needed for better Tor support. Tor is already available and works just fine on Haiku when I use it in Qt applications. It would be nice if I could use more native applications without telling every remote server my real IP address.