Std domain: don't expect "refprogram" attribute on an xref node.

This would lead to an exception if we pass in some generic x-ref node
that tries to resolve as an option.

sphinx/domains/std.py

@@ -600,7 +600,7 @@ class StandardDomain(Domain):
             return make_refnode(builder, fromdocname, docname,
                                 labelid, contnode)
         elif typ == 'option':
-            progname = node['refprogram']
+            progname = node.get('refprogram', '')
             docname, labelid = self.data['progoptions'].get((progname, target),
                                                             ('', ''))
             if not docname: