Update Maintainers.PL for divergence from cpan

I've noted some files customized in blead and marked
Devel::PPPort as 'undef' upstream as it appears unmaintained
and rjbs has asked the maintainer to switch it to blead.

With this change, core-cpan-diff on the 'cpan' upstream is clean.
t/porting/customized.t: consistent data file sorting