Clean up code paths around proxy PAC configuration.

The prior implementation of retrieving a PAC file from a URL didn't work.
A fix is coming soon. However, supposing it actually retrieved a script,
everything else worked as expected.

Removed the automatic addition of the direct endpoint in the case of no
discovered proxies. This was too graceful -- if the proxy APIs returned
a proxy config but we failed for some reason (bad PAC URL, for instance),
then we should flat-out fail the request. It turns out the proxy APIs
do return kCFProxyTypeNone in cases where they want you to use a direct
connection, so we obey that now.

Test cases observed:
HTTPS proxy configured: no direct config returned.
PAC URL configured: direct config returned last after discovered ones.
No proxy configuration: direct config returned.