Enabling cwrappers auto-detection

[Jonathan Perkin <jperkin%joyent.com@localhost>]

As discussed, we would like to enable cwrappers for platforms where we
know it works and there is a real benefit from using them.  I've
written up a patch here:

  https://gist.github.com/jperkin/1e6c26cb605945cdc03402b61851c61b

which enables them by default for the platforms I have been running
them on for a very long time now (Darwin/Linux/SunOS).  Users can
still opt in or out, this just changes the default to auto-detect.

I'll get this in shortly unless there are major objections, and then
other platform maintainers can enable their platforms as necessary by
setting _OPSYS_SUPPORTS_CWRAPPERS=yes in their mk/platform file.

Cheers,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com