Correct type checking for Module#{append,prepend}_features.
Tag prepend tests from MRI that are not yet passing.
Fixes for missing and incorrect spawn logic.

* Native pipe FDs should be set cloexec so they close
  automatically during a spawn or exec.
* Non-sh-using spawn logic was incomplete and did not pass any
  posix_spawn flags or env values to posix_spawn.
Updates and fixes for Process::Status

* Define best-guess equivalents to W macros for process status
* Use real native status in Proces::Status
* Re-port existing Status methods to use native status
* Add missing #signame method
* Cleanup and reorg Process::Status code
Restore this.isModule checks in {append,prepend}_features.
Mask a test that needs NoMemoryError to be reliably thrown.
Bump up timeout on a process-launching test for JRuby.