implement socks5h proxy support for http method

Socks support is a requested feature in sofar that the internet is
actually believing Acquire::socks::Proxy would exist. It doesn't and
this commit isn't adding it as that isn't how our configuration works,
but it allows Acquire::http::Proxy="socks5h://…". The HTTPS method was
changed already to support socks proxies (all versions) via curl. This
commit implements only SOCKS5 (RFC1928) with no auth or pass&user auth
(RFC1929), but not GSSAPI which is required by the RFC. The 'h' in the
protocol name further indicates that DNS resolution is delegated to the
socks proxy rather than performed locally.

The implementation works and was tested with Tor as socks proxy for
which implementing socks5h only can actually be considered a feature.

Closes: 744934