scripts with stub.
* ruby.c (load_file_internal): assume xflag for exe file as well
  as no-shebang file.
* tool/rbinstall.rb: install script programs.
* win32/mkexports.rb (Exports#initialize): alias ruby_sysinit for
  stub.
* win32/stub.c: stub for scripts.  [EXPERIMENTAL]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@29738 b2dd03c8-39d4-4d8f-98ff-823fe69b080e