pass errno from exec() to parent in spawn()

pass errno from exec() to parent in spawn()

  Open a pipe, set FD_CLOEXEC.

  Child: if exec() fails, grab errno and write it to pipe.

  Parent: try to read from the pipe -- if you get something, it means the
  child didn't exec and the reason is in the pipe. Wait for the child to exit
  and return -1 and set errno to whatever the child got.

  Also use _exit() instead of exit() when dying in the child after exec
  failure -- running exit hooks there would probably be bad.

  (Somewhat edited from Nicolas' original patch.)

Signed-off-by: Nikodemus Siivola <nikodemus@random-state.net>