x/net/proxy: does not support dialing with a given Context

The x/net/proxy package doesn't support dialing a proxy with a given Context.

This also impacts the net/http package because a Client can dial through a socks5 proxy if specified (usualy through ProxyFromEnvironment). If the socks5 proxy blocks before connecting the socket, the http Client could get stuck without a way to abort the connection by cancelling a Context.