Blank the screen with color 0, add third color for failed logins

Blank the screen with color 0, add third color for failed logins

- Adds another color in config.def.h, COLOR_INIT
- Renames the colours from numerical ones to ones with meaningful names;
  COLOR_INPUT for when there is content in the input buffer and COLOR_EMPTY
  for when the input buffer has been cleared (backspaced or a failed attempt).
- Ensures XFreeColors frees the right number of colours. This is now derived
  from the size of `Lock->colors` rather than being an integer literal.
- Makes slock exhibit the behaviour described by Markus

The default colours are the same as the ones slock currently uses, with the
exception of the new color, which I have set to red, as it indicates someone
has either failed an attempt to unlock, or that they have entered input and
erased it all.