Bug 13028: Prevent potential proxy bypass cases.

It looks like these cases should only be invoked in the NSS command line
tools, and not the browser, but I decided to patch them anyway because there
literally is a maze of network function pointers being passed around, and it's
very hard to tell if some random code might not pass in the proper proxied
versions of the networking code here by accident.

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1433509