Make extended commands return defined flags

Instead of returning 0 or 1, we'll now use ECMD_OK or ECMD_TURN.
These have the same meaning as the hardcoded numbers; ECMD_TURN
means the command uses a turn.

In future, could add eg. a flag denoting "user cancelled command"
or "command failed", and should clear eg. the cmdq.

Mostly this was simply replacing return values with the defines
in the extended commands, so hopefully I didn't break anything.

include/func_tab.h

@@ -18,7 +18,7 @@
 struct ext_func_tab {
     uchar key;
     const char *ef_txt, *ef_desc;
-    int (*ef_funct)(void);
+    int (*ef_funct)(void); /* must return ECMD_foo flags */
     int flags;
     const char *f_text;
 };