sslutil: expose attribute indicating whether SNI is supported

This will be used so clone bundles can advertise whether URLs require
SNI. This will be explained more in a subsequent patch.

mercurial/sslutil.py

@@ -16,6 +16,8 b' import sys'
 from .i18n import _
 from . import error, util
 
+hassni = getattr(ssl, 'HAS_SNI', False)
+
 _canloaddefaultcerts = False
 try:
     ssl_context = ssl.SSLContext