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
Because no flags were set for regex matching, the regex engine was
assuming that the subject string past-the-end matched a end-of-line.
As the subject string already ended with a \n character, the regex
engine processing of the "past-the-end" position would match '^$'
as ^ matched past the existing \n and $ matched the assumed
end-of-line.
Fixes#3799
