Update mlterm to 3.1.9. (leaf package minor update)

Changes per doc/en/ReleaseNote:

ver 3.1.9
* Remove --enable-sixel option from configure, and add --disable-image option.
  That is, support sixel graphics by default unless configure with
  --disable-image option. (tool/w3m-remoteimg which doesn't depend on sixel
  graphics but on OSC 5379 show_picture also works on mlterm by default.)
* Separate SSH client functions to a dynamically loaded module (libptyssh)
  in platforms except win32.
* Treat key strings of "main" configuration file case-insensitively.
  (Both encoding=utf8 and ENCODING=utf8 are available.)
* Assign -M and -P options as the short options of --im and --clip.
* Add "allow_change_shortcut" (--shortcut) option which enables "set_shortcut"
  command of configuration protocol.
* Bug fixes:
  Fix the conflict of DCS(0x90) and the first byte of SJIS, GBK and so on.
  Fix the bug which disabled input and output of pty when configuration program
  such as mlconfig is started.
  Encode input character masked with 0x80 to the character encoding of pty
  if the value of mod_meta_mode option is 8bit. (Thanks to Vladimir Elisseev)
  Fix the bug which disabled Alt+symbol and Control+@ in win32.
  Fix the bug which disabled some shortcut key settings.


(tsutsui)