Use floating pane instead of popup on tmux 3.7 or above (#4850) (#4852)

Unlike a popup, a floating pane is not modal; you can switch to other
panes and windows while fzf is running, move and resize the pane with
the mouse, zoom it to fullscreen, and use copy-mode in it.

- Floating pane always has a native border, so 'border-native' is
  implied; give new 'border-fzf' option to fall back to a popup where
  fzf draws its own border
- Popup is also used on tmux versions below 3.7, or when the window is
  too small to fit a floating pane
- new-pane does not block until the command finishes and does not
  propagate the exit status; block on a wait-for channel signaled by
  the pane and pass the exit status through a temporary file
- Watchdog process signals the channel when the pane is closed
  abnormally (e.g. kill-pane)
- Kill the pane when the proxy process is interrupted, like a popup
  dying with its client
- Unzoom the window before creating the floating pane; doing so over a
  zoomed window crashes the tmux server on 3.7b, and newer versions of
  tmux unzoom the window anyway
- Floating pane size excludes the border and the position is that of
  the content area; treat the requested size as the total footprint
  including the border for consistency with popups
- Close the pane on exit even when remain-on-exit is on
- Pre-create the exit status file with O_EXCL to prevent tampering on
  a shared TMPDIR

CHANGELOG.md

@@ -3,6 +3,13 @@ CHANGELOG
 
 0.74.0 (WIP)
 ------------
+- On tmux 3.7 or above, `--popup` starts fzf in a floating pane instead of a popup (#4850)
+    - Unlike a popup, a floating pane is not modal; you can switch to other panes and windows while fzf is running, move and resize the pane with the mouse, zoom it to fullscreen, and use copy-mode in it
+    - A floating pane always has a native border, which is what makes the pane movable and resizable, so `border-native` is implied
+    - Give `border-fzf` to fall back to a popup where fzf draws its own border
+      ```sh
+      fzf --popup center,80%,border-fzf
+      ```
 - Added `result-final` event, a variant of `result` that is not triggered while the input stream is still open (#4835)
     - Use it for one-shot, per-query actions that would otherwise re-fire on every intermediate snapshot during loading
       ```sh