sslutil: refactor code for fingerprint matching We didn't need to use a temporary variable to indicate success because we just return anyway. This refactor makes the code simpler. While we're here, we also call into formatfingerprint() to ensure the fingerprint from the proper hashing algorithm is logged.

sslutil: print SHA-256 fingerprint by default The world is starting to move on from SHA-1. A few commits ago, we gained the ability to define certificate fingerprints using SHA-256 and SHA-512. Let's start printing the SHA-256 fingerprint instead of the SHA-1 fingerprint to encourage people to pin with a more secure hashing algorithm. There is still a bit of work to be done around the fingerprint messaging. This will be addressed in subsequent commits.

sslutil: move and change warning when cert verification is disabled A short time ago, validatesocket() didn't know the reasons why cert verification was disabled. Multiple code paths could lead to cert verification being disabled. e.g. --insecure and lack of loaded CAs. With the recent refactorings to sslutil.py, we now know the reasons behind security settings. This means we can recognize when the user requested security be disabled (as opposed to being unable to provide certificate verification due to lack of CAs). This patch moves the check for certificate verification being disabled and changes the wording to distinguish it from other states. The warning message is purposefully more dangerous sounding in order to help discourage people from disabling security outright. We may want to add a URL or hint to this message. I'm going to wait until additional changes to security defaults before committing to something.

sslutil: add devel.disableloaddefaultcerts to disable CA loading There are various tests for behavior when CA certs aren't loaded. Previously, we would pass --insecure to disable loading of CA certs. This has worked up to this point because the error message for --insecure and no CAs loaded is the same. Upcoming commits will change the error message for --insecure and will change behavior when CAs aren't loaded. This commit introduces the ability to disable loading of CA certs by setting devel.disableloaddefaultcerts. This allows a testing backdoor to disable loading of CA certs even if system/default CA certs are available. The flag is purposefully not exposed to end-users because there should not be a need for this in the wild: certificate pinning and --insecure provide workarounds to disable cert loading/validation. Tests have been updated to use the new method. The variable used to disable CA certs has been renamed because the method is not OS X specific.

sslutil: store flag for whether cert verification is disabled This patch effectively moves the ui.insecureconnections check to _hostsettings(). After this patch, validatesocket() no longer uses the ui instance for anything except writing messages. This patch also enables us to introduce a per-host config option for disabling certificate verification.

sslutil: remove "strict" argument from validatesocket() It was only used by mail.py as part of processing smtp.verifycert, which was just removed.