Add support for a third color in face definition that controls
the underline and a 'c' attribute for curly underline (that takes
precedence over 'u' if both are specified)
Allow empty colors to mean default, so that `,,red+u` means the
same as `default,default,red+u`
Fixes#4138
Ranges specified with a +<length> were inconsistent, with +0 meaning
an empty range, while +1 meant a two character long range (first character
+ the following one). Change that to mean a single character.
Fixes#3479