patch 9.1.0171: Small split-move related improvements

Problem:  small improvements can be made to split-move related
          functions.
Solution: apply them (Sean Dewar):

- Improve some doc comments (frame_flatten should still work for non-current
  tabpages, despite the topframe check, which looks benign, though I'm unsure if
  it's still needed; see #2467).

- f_win_splitmove should check_split_disallowed on wp, not targetwin, as that's
  what win_splitmove checks (though it's probably unnecessary to check
  b_locked_split at all; see #14109, which I hope to get around to finishing at
  some point).

- Make winframe_restore restore window positions for the altframe, which
  winframe_remove changes. This doesn't affect the prior behaviour, as we called
  win_comp_pos after, but as win_comp_pos only works for curtab, and
  winframe_remove supports non-current tabpages, we should undo it. Regardless,
  this should mean we don't need win_comp_pos anymore; adjust tests to check
  that window positions remain unchanged.

  I'm not sure win_comp_pos is needed after last_status anyway if it doesn't
  steal rows from another frame to make room for a new statusline, which
  shouldn't be the case after winframe_remove? To be safe, I'll leave it as is.

closes: #14185

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    171,
 /**/
     170,
 /**/