Cleaner OPSYS "features" support

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

I recently changed the OSX 10.6 builds to use a newer GCC, and that
has now exposed the lack of strnlen on that platform.  After manually
patching a few packages I figured it'd be a lot better to simply
extend our existing "features" infrastructure to support it.

I really dislike the OPSYS hardcoding in mk/features/features-vars.mk
so have started to extrapolate the information correctly into
mk/platform with the addition of strnlen.  The commit is here:

  https://github.com/joyent/pkgsrc/commit/20e52a141688ffa3ce341f52050242621e585996

It's not enough of an issue to go in before the freeze, this is just a
quick review request for the general approach and I'll work on
extending it further before committing after the freeze.

I'd also like to change libnbcompat to be a standalone package rather
than the inplace stuff, as the more we use it the more this costs.  If
there are things to watch out for when doing this let me know.

Thanks,

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