sslutil: require serverhostname argument (API)

All callers now specify it. So we can require it.

Requiring the argument means SNI will always work if supported
by Python.

The main reason for this change is to store state on the socket
instance to make the validation function generic. This will be
evident in subsequent commits.

mercurial/sslutil.py

@@ -120,6 +120,9 b' def wrapsocket(sock, keyfile, certfile, '
       server (and client) support SNI, this tells the server which certificate
       to use.
     """
+    if not serverhostname:
+        raise error.Abort('serverhostname argument is required')
+
     # Despite its name, PROTOCOL_SSLv23 selects the highest protocol
     # that both ends support, including TLS protocols. On legacy stacks,
     # the highest it likely goes in TLS 1.0. On modern stacks, it can