Split get_environ_proxies into two methods.

This makes it possible to get at the no_proxy logic separately.
Move auth rebuild to its own method.
Add method for rebuilding proxy configuration.

This includes auth.
Remove some extraneous comments to please KR.
Test stripping standard auth.
Style changes thanks to @sigmavirus24.
Merge pull request #1951 from Lukasa/proxyauth

Re-evaluate proxy authorization.