Backports and portability rollbacks

Backports and portability rollbacks

1f8a0b38638b171cf789a9f44cc9e8cd38bbf4d3:
    All extensions are at the top level, so no longer any need to recurse.

abd7186c6748d276ced6dd10e94e6c04ba10b768:
    Remove special casing for extensions nested inside other extensions.
    (Incorporates "[PATCH] Don't include Hash::Utils::FieldHash twice" from
    Jerry Hedden.)

238a6851e196b122761952f8b2127d8fe9e01658:
    Cope with new-style ext/Data-Dumper as well as old-style ext/Data/Dumper

aefe3e7eb5caf2308d0c098e045a537a24dc42b6:
    Exclude VMS extensions on non-VMS.