Fix application name when printing subcommand help.

I introduced this bug in PR  #5754. It's fixed against master in  #5796,
but that won't be backported to 2.x.

IPython/config/application.py

@@ -326,7 +326,7 b' class Application(SingletonConfigurable):'
         lines.append('-'*len(lines[0]))
         lines.append('')
         for p in wrap_paragraphs(self.subcommand_description.format(
-                    app=os.path.basename(self.argv[0]))):
+                    app=self.name)):
             lines.append(p)
             lines.append('')
         for subc, (cls, help) in iteritems(self.subcommands):