* string.c: refactoring, especially about string flags.
* string.c (STR_UNSET_NOCAPA): removed.
  Use FL_UNSET() with STR_SHARED.
* string.c (rb_str_capacity): check STR_SHARED directly
  beacuse it is not a embed string.
* string.c (rb_str_modify_expand): ditto.
* string.c (rb_str_shared_replace): use STR_SET_SHARED().
* string.c (str_make_independent_expand): remove STR_UNSET_NOCAPA()
  because `str' is not shared string.



git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44825 b2dd03c8-39d4-4d8f-98ff-823fe69b080e