Null check the status value.
Add boundscheck.
Oops...don't lose command when doing chdir.
Fix test_backquote logic to match MRI and fix JRuby to pass.

* posix_spawnp was not returning -1 on error (returning bogus pid
  instead). Fixed in jnr-posix.
* All places we call posix_spawnp need to check for error and save
  errno in PopenExecutor for future use.
Cleanup.