Reorganize proxy resolution, and support SOCKS and other proxy types

Reorganize proxy resolution, and support SOCKS and other proxy types

Push the proxy resolution code from SoupSession down into
SoupConnection and SoupSocket. If using a SoupProxyResolverDefault,
just enable proxy support on the GSocketClient instead (after adding
"http" as an application protocol). This way we get support for SOCKS
proxies (and any other proxies supported by GProxy types).

https://bugzilla.gnome.org/show_bug.cgi?id=553269