regex: Convert regnode FLAGS fields to charset enum

regex: Convert regnode FLAGS fields to charset enum

The FLAGS fields of certain regnodes were encoded with USE_UNI if
unicode semantics were to be used.  This patch instead sets them to the
character set used, one of the possibilities of which is to use unicode
semantics.  This shortens the code somewhat, and always puts the
character set in the flags field, which can allow use of switch
statements on it for efficiency, especially as new values are added.