Add support for path-based client authentication

It seems surprising to return a formatted string here:

+/**
+ * Fetches the client authentication path.
+ */
+function hosting_https_client_authentication_path($node) {
+  if (empty($node->https_client_authentication_path)) {
+    return t('<em>(global)</em>');
+  }
+  return filter_xss($node->https_client_authentication_path);
+}

This looks weird. Are we really concatenating a field name?

-    $result = db_query("SELECT https_enabled$client_auth_column FROM {hosting_https_site} WHERE vid = :vid",
+    $result = db_query("SELECT https_enabled$client_auth_enabled_column$client_auth_path_column FROM {hosting_https_site} WHERE vid = :vid",

This looks like a typo ("CLient"):

   $lines[] = "  SSLVerifyCLient optional_no_ca";