url: fix unix-socket support for proxy-disabled builds

Prior to this change if curl was built with Unix Socket support
(--enable-unix-sockets) and without Proxy support (--disable-proxy) then
unix socket options would erroneously be ignored.

Regression introduced in:
0b8d682f

Bug: https://github.com/curl/curl/issues/1274


Reported-by:  <mccormickt12@users.noreply.github.com>

Closes https://github.com/curl/curl/pull/1289