PREFER.pkg defaults

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

In response to Richard's change of PREFER.openssl for SunOS, I'd like
to introduce a new private variable _OPSYS_PREFER.<pkg> which will
allow us to provide an advisory per-platform default.

Currently the mk/platform files set PREFER.<pkg> which has the highest
precedence, so users cannot override these defaults via PREFER_PKGSRC
or PREFER_NATIVE.  I think it's messy having users put PREFER.<pkg> in
mk.conf, especially as it is occasionally overridden in pkgsrc.

The patch is here:

  http://www.netbsd.org/~jperkin/opsys-prefer.patch

and I've also taken the opportunity to expand the documentation.  With
this, the SunOS case where a PREFER_PKGSRC user wants to use the
builtin openssl would be handled with:

  PREFER_PKGSRC=        yes
  PREFER_NATIVE=        openssl

in mk.conf, which to me is most in line with the principle of least
astonishment.

Thanks,

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