Merge branch 'refs/heads/t/gui-improvements'

Improved wide-character support and fixes related to signal
handling. This topic branch was cooking in next for two months.

* refs/heads/t/gui-improvements:
  gui.c: Constify argument to find_cmd_byname().
  gui.c: Improve description of signal task.
  gui.c: Remove pointless return statement.
  gui.c: Remove silly warning on SIGINT.
  gui: Kill process group *before* shutting down curses.
  gui.c: Reset terminal on shutdown in external mode.
  gui: Avoid bad terminal state with xterm.
  gui.c: Constify local variables of add_spaces().
  Introduce sanitize_str().
  string.c: Simplify and rename wide character helpers.