This allows proxy URLs such as:
http://azurediamond:hunter2@192.168.122.154:38274
to be used in order to dial out to control, logs or derp servers.
Signed-off-by: Christine Dodrill <xe@tailscale.com>
Otherwise when PAC server is down, we log, and each log entry is a new
HTTP request (from logtail) and a new GetProxyForURL call, which again
logs, non-stop. This is also nicer to the WinHTTP service.
Then also hook up link change notifications to the cache to reset it
if there's a chance the network might work sooner.
