Note.  I have made more changes based on feedback from the mailing list.

1) removed use of scm_c_take_typed_bytevector
2) changed code to generate PAGE_SIZE

I think the mmap finalizer still needs review from the Guile experts.

I'm attaching a patch to the v3.0.9 release (commit 9b20ca).
Sorry about not using format-patch, but diff.

Matt