move struct propname to a header file

In doing so, avoid repeated layout declaration in cmd.c

include/timeout.h

@@ -60,4 +60,9 @@ typedef struct fe {
     Bitfield(needs_fixup, 1); /* does arg need to be patched? */
 } timer_element;
 
+struct propname {
+    int prop_num;
+    const char *prop_name;
+};
+
 #endif /* TIMEOUT_H */