parse.y: simplify

* parse.y (f_label): extract from f_kw and f_block_kw.

* parse.y (f_opt, f_block_opt): use f_norm_arg to simlify actions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42563 b2dd03c8-39d4-4d8f-98ff-823fe69b080e