Final distfile argmax fix

[Jonathan Perkin <jperkin%mnx.io@localhost>]

A while back I posted a patch to correctly handle wip/grafana which now 
has so many distfiles it breaks both make and checksum.awk.

I've revisited the patch, cleaned it up a bit further, made it more 
generic, and also made it work with the mktool patch for fetch, as that 
also now requires temporary files if using an older make.

The current patch for review is here:

  https://github.com/NetBSD/pkgsrc/commit/fa88ed98c9db92f637ce54ce71c85cfc253063a3

I've tested this on macOS with new make, and on NetBSD with both old 
native make and newer bmake, and with/without mktool, but would 
appreciate a bunch more testing.  I will of course push it through a 
full bulk build too.

Please review and test.

Thanks,

--
Jonathan Perkin   -   mnx.io   -   pkgsrc.smartos.org
Open Source Complete Cloud   www.tritondatacenter.com