configitems: register the 'web.ipv6' config

mercurial/configitems.py

@@ -625,6 +625,9 b" coreconfigitem('web', 'encoding',"
 coreconfigitem('web', 'errorlog',
     default='-',
 )
+coreconfigitem('web', 'ipv6',
+    default=False,
+)
 coreconfigitem('worker', 'backgroundclose',
     default=dynamicdefault,
 )