Add support for the $unit scope identifier.

This relies on using the automatically generated name for the unit scope.
This could collide with an escaped identifier, so it would be better to
identify the scope by a reference to the PPackage object, but for now,
do it the easy way.

lexor.lex

@@ -429,6 +429,12 @@ TU [munpf]
 
       if (strcmp(yytext,"$attribute") == 0)
 	    return KK_attribute;
+
+      if (gn_system_verilog() && strcmp(yytext,"$unit") == 0) {
+	    yylval.package = pform_units.back();
+	    return PACKAGE_IDENTIFIER;
+      }
+
       yylval.text = strdupnew(yytext);
       return SYSTEM_IDENTIFIER; }