The API here is similar enough that this is an easier match than
trying to refit our ShellLauncher for these other forms. This
change allows the sassc gem extension build to succeed without any
native support loaded (or without native process support as on
Windows).

This was changed sometime between 8 and 11 from UNIXProcess to
ProcessImpl. Note that while this exposes the proper class, it
still will not work without opening java.base/java.lang to JRuby.

Channel is intended to be an unbuffered IO construct backed
directly by a native file descriptor, but it has subclasses that
wrap other types of IO streams. In the case of OutputStream, the
wrapper seems to have a crucial flaw: it does not flush the
underlying stream. Since Channel provides no way to flush, the
buffered data will sit there forever.

This change modifies our logic to use a "sync" version of the
OutputStream channel wrapper, forcing the underlying stream to
flush after each write operation.

These fixes bring test_open3 down to only two hanging specs, which
hang due to the inablity to pass raw file descriptors through
ProcessBuilder into the eventual subprocess.

* Properly propagate options through all methods.
* popen2 should inherit stderr from parent.
* The wait thread should mimic a Process.detach thread with pid
  method and thread-local value.
* The process exit status should be shifted right 8 bits to mimic
  native packed process results.
* Move Open3.popen3 to using this new logic rather than the
  one-off custom IO.popen3 we implemented years ago. The new
  logic behaves better.
* Avoid kwrest arguments since they behave poorly in Ruby 2.6 and
  earlier.
* Set process streams to sync. This works together with 5a6912c
  to ensure that the wrapped OutputStream does not buffer data.