Log Message:
Update mlterm to 3.1.8.

Changes per doc/en/ReleaseNote:

ver 3.1.8
* Support framebuffer on FreeBSD. (Experimental)
* 'key' configuration file accepts Button1 - Button5.
* Remove "conf_menu_path_1", "conf_menu_path_2", \ 
"conf_menu_path_3" and
  "button3_behavior" options which are integrated to shortcuts.
  [Migration]
  (~/.mlterm/main)        (~/.mlterm/key)
  conf_menu_path_1=... => Control+Button1="menu:..."
  conf_menu_path_2=... => Control+Button2="menu:..."
  conf_menu_path_2=... => Control+Button3="menu:..."
  button3_behavior=... => Button3="exesel:..."
* Add "set_shortcut" command to configuration protocol.
* Bug fixes:
  Fix the bug of DECCRA.
  Fix the bug which broke input string of "ExecCmd" field of the \ 
connection dialog in win32.
  Fix the bug which didn't redraw a part of full-width characters when window is \ 
exposed.
  Enable to change "vertical_mode" option dynamically in framebuffer.