[PATCH 08 of 11 V4] sslutil: config option to specify TLS protocol version

[timeless]

I'm not a huge fan of this abort:
+    # Allow minimum TLS protocol to be specified in the config.
+    def validateprotocol(protocol, key):
+        if protocol not in configprotocols:
+            raise error.Abort(
+                _('unsupported protocol from hostsecurity.%s: %s') %
+                (key, protocol),
+                hint=_('valid protocols: %s') %
+                     ' '.join(sorted(configprotocols)))

It means that when a system updates to hg-default that supports this,
and I later build a newer hg that supports 1.3 or something, I'll
start getting aborts when using the system default, which means I'll
have to spend lots and lots of time trying to coerce the system
version to work at all.

Having to have two configs per repo or similar is a recipe for pain,
one to which I do not look forward.

We've had similar problems in the past -- the broken versions of
mercurial which can't safely handle the obsolete store.