ui: add an instance flag to hold --insecure bit

Currently, when --insecure is used we set web.cacerts=! and
socket validation takes this value into account. web.cacerts=!
is not documented AFAICT and is purely an internal implementation
detail.

Let's be more explicit about what is going on by introducing a
dedicated variable outside of the config values to track that
--insecure is used.

mercurial/ui.py

@@ -107,6 +107,8 b' class ui(object):'
         self._trustusers = set()
         self._trustgroups = set()
         self.callhooks = True
+        # Insecure server connections requested.
+        self.insecureconnections = False
 
         if src:
             self.fout = src.fout
@@ -120,6 +122,7 b' class ui(object):'
             self._trustgroups = src._trustgroups.copy()
             self.environ = src.environ
             self.callhooks = src.callhooks
+            self.insecureconnections = src.insecureconnections
             self.fixconfig()
         else:
             self.fout = sys.stdout