menuconfig: Increase indent and make Unicode more robust

This commit adds the following changes from upstream:

  - ed38e895ace ("Increase indent for implicit submenus to 4")

    Suggested by Carles Cufí. Makes it easier to see the menu structure
    at a glance.

  - 1d252b30c77 ("menuconfig: Convert the C locale to a UTF-8 locale for
                  LC_CTYPE")

    Makes Unicode input work on many systems with bad defaults.

    Also fixes some interface ugliness, like down arrows turning into
    upside-down T's.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>