Set NBPAX_PROGRAM_PREFIX?=nb by default, avoids problems on various platforms
where software assumes features of the native 'tar' and break when 'tar' is
the NetBSD version.

We are too close to the pkgsrc-2012Q2 branch to remove NBPAX_PROGRAM_PREFIX
completely, but if it's apparent that no platforms need to override this
default then it will be removed completely for the next branch.

(jperkin)