merge with stable

sslutil: try to find CA certficates in well-known locations Many Linux distros and other Nixen have CA certificates in well-defined locations. Rather than potentially fail to load any CA certificates at all (which will always result in a certificate verification failure), we scan for paths to known CA certificate files and load one if seen. Because a proper Mercurial install will have the path to the CA certificate file defined at install time, we print a warning that the install isn't proper and provide a URL with instructions to correct things. We only perform path-based fallback on Pythons that don't know how to call into OpenSSL to load the default verify locations. This is because we trust that Python/OpenSSL is properly configured and knows better than Mercurial. So this new code effectively only runs on Python <2.7.9 (technically Pythons without the modern ssl module).

sslutil: issue warning when unable to load certificates on OS X Previously, failure to load system certificates on OS X would lead to a certificate verify failure and that's it. We now print a warning message with a URL that will contain information on how to configure certificates on OS X. As the inline comment states, there is room to improve here. I think we could try harder to detect Homebrew and MacPorts installed certificate files, for example. It's worth noting that Homebrew's openssl package uses `security find-certificate -a -p` during package installation to export the system keychain root CAs to etc/openssl/cert.pem. This is something we could consider adding to setup.py. We could also encourage packagers to do this. For now, I'd just like to get this warning (which matches Windows behavior) landed. We should have time to improve things before release.