- Changing the global function ok_for_lower_case_names()
  into a method in class Lex_ident_fs.

- Changing a few functions/methods to get the database name
  as a "const LEX_CSTRING" instead of a "const char *".
  All these functions/methods use ok_for_lower_case_names()
  inside. This change helps to avoid new strlen() calls, and also
  removes a few old strlen() calls.