Update Porting/release_managers_guide.pod and Porting/makerel

Update Porting/release_managers_guide.pod and Porting/makerel

In the course of releasing perl-5.11.5 I found a few things that are not
quite correct or no longer relevant:

- Add comments on the problems with doing this on Win32
- There are no longer Roman numerals in README.vms (since eb505c2afb6b5ac2aaba7935878ae6494d44732a)
- Note that Module::CoreList's Changes and META.yml files need updating too
- Fix Porting/makerel's null device redirection

One bigger problem remains to be resolved: when run in a normal cmd.exe
shell on Win32, the Porting/makerel script currently generates tarballs
containing world-writable files and directories, which the PAUSE indexer
then rejects as a possible security risk.