vconsole: add two new toggle functions, remove old enable/disable ones

      Add toggle_utf8() and toggle_utf8_sysfs() and use them in place of old
enable/disable functions. toggle_utf8() also adds iutf8 setting and is
set up to be called per-console (in subsequent patches).

Note, that old disable_utf8() didn't bother checking if it was ok
to change the kbdmode.