flags.

* re.c (rb_reg_initialize_m): allow nil as third argument and
  ignore, and mask code flags if the argument is given.
  [ruby-dev:20885]

* re.c (rb_reg_options): get common flags directly.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@4155 b2dd03c8-39d4-4d8f-98ff-823fe69b080e