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.

mail: unsupport smtp.verifycert (BC) smtp.verifycert was accidentally broken by cca59ef27e60. And, I believe the "loose" value has been broken for longer than that. The current code refuses to talk to a remote server unless the CA is trusted or the fingerprint is validated. In other words, we lost the ability for smtp.verifycert to lower/disable security. There are special considerations for smtp.verifycert in sslutil.validatesocket() (the "strict" argument). This violates the direction sslutil is evolving towards, which has all security options determined at wrapsocket() time and a unified code path and configs for determining security options. Since smtp.verifycert is broken and since we'll soon have new security defaults and new mechanisms for controlling host security, this patch formally deprecates smtp.verifycert. With this patch, the socket security code in mail.py now effectively mirrors code in url.py and other places we're doing socket security. For the record, removing smtp.verifycert because it was accidentally broken is a poor excuse to remove it. However, I would have done this anyway because smtp.verifycert is a one-off likely used by few people (users of the patchbomb extension) and I don't think the existence of this seldom-used one-off in security code can be justified, especially when you consider that better mechanisms are right around the corner.