native PBC issues

[Leopold Toetsch]

First of all its of course my fault. This whole stuff is vastly untested 
and it makes my brain hurt. I've cleaned up a bit and committed some 
changes, but this might still all be wrong.

So to get that running, people please submit native_pbc test files from 
64-bit machines and from big-endian architectures. Please follow the 
instructions in t/native_pbc/*.t and commit test files with -kb.

Reading foreign PBC isn't number 1 priority yet, but it should work some 
time.

Thanks,
leo