Change special keys into extended commands

Changes most of the special keys used in the main input loop
into extended commands:

- movement keys are now bound to extended commands, eg.
  #movewest and so on.

- m-prefix is now #reqmenu extended command, still bound to
  the 'm' key.

- run, rush, and fight are now extended commands, still bound
  to the same keys as previously.

- nopickup and runnopickup keys are removed.
  Nopickup was using 'm' key, the same as the m-prefix, so
  allow #reqmenu to modify movement commands to disable pickup.

- multiple prefix commands are allowed. This lets user to
  use #reqmenu, followed by #run, followed by movement to simulate
  runnopickup behaviour. (If necessary, adding runnopickup back
  as an extended command would be easy)

src/dig.c

@@ -987,13 +987,8 @@ use_pick_axe(struct obj *obj)
     downok = !!can_reach_floor(FALSE);
     dsp = dirsyms;
     for (dir = 0; dir < N_DIRS_Z; dir++) {
-        char dirch;
-        if (dir == DIR_DOWN)
-            dirch = cmd_from_func(dodown);
-        else if (dir == DIR_UP)
-            dirch = cmd_from_func(doup);
-        else
-            dirch = g.Cmd.move[dir];
+        char dirch = cmd_from_dir(dir, MV_WALK);
+
         /* filter out useless directions */
         if (u.uswallow) {
             ; /* all directions are viable when swallowed */