Use posix_spawn_file_actions for stap_system_read

Use posix_spawn_file_actions for stap_system_read

The file actions can handle the dup2 necessary to hook a pipe onto the
child's stdout.  It could do the necessary closing too, but we'll just
use FD_CLOEXEC for that.

* util.cxx (stap_spawn): Take file_actions for the posix_spawn.
  (pipe_child_fd): New, open a pipe and set the dup2 action for it.
  (stap_system_read): Let stap_spawn and pipe_child_fd do the hard part.