mlterm: update to 3.8.6.

Changes noted in doc/en/ReleaseNote:

ver 3.8.6
* Support preserveAspectRatio option of OSC 1337.
* Support DECRQCRA, CSI 15 t, CSI 16 t, CSI 19 t and OSC 5;Ps;? ST.
* Support framebuffer on FreeBSD(98).
* Add vte 0.52.0 API symbols to libvte compatible library.
* Add --rv / "rv_color" option.
* Add servman.exe which manages the list of servers on win32.
* Add "PrivKey" entry to the ssh connection dialog on win32 and Android.
* Add "SCP" entry to the context menu on Android.
* Drop --servlist / "server_list" option.
* Improve performance of redrawing screen when type_engine=cairo.
* Show box style cursor when the cursor is on a picture or an emoji.
* Modify DECRC to save/restore last-column flag used to control wrapping behavior
  rather than manipulating DECAWM (See https://github.com/mattiase/wraptest and
  https://invisible-island.net/xterm/xterm.log.html#xterm_328)
* Change the default value of ssh_private(public)_key option from
  /sdcard/.ssh/id_rsa(.pub) to /sdcard/.mlterm/id_rsa(.pub) on Android.
* Bug fixes:
  Fix compiling error if ./configure is executed with --with-gui=fb,xlib option.
  Fix disappearing title bar on 'lxde' window manager.
  Fix a bug which disabled to replace system palettes by colors of a wall picture
  (if "use_ansi_colors" option is disabled and visual depth is 4) since 3.8.4.
  Fix the response strings of OSC 20 and OSC 21.
  Use the background color specified by SGR instead of the default one to draw
  pictures and emojis.

(tsutsui)