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
Using <fg>,<bg>+<attr>@<base> will apply the given fg color,
bg color and attributes on top of base dynamically. Simply giving
<base> is a shorthand for default,default@<base>.
Inspired by the discussion in #2862
