parse_proxy: treat "socks://x" as a socks4 proxy

Selected socks proxy in Google's Chrome browser. Resulting in the
following environment variables:

NO_PROXY=localhost,127.0.0.0/8
ALL_PROXY=socks://localhost:1080/
all_proxy=socks://localhost:1080/
no_proxy=localhost,127.0.0.0/8

... and libcurl didn't treat 'socks://' as socks but instead picked HTTP
proxy.

Reported by: Scott Bailey

Bug: http://curl.haxx.se/bug/view.cgi?id=3566860