Add test for mmap using Parrot fd unnecessarily.
whitespace
Allow Parrot to close unneeded mmap fd.

If Parrot does not need to write through any changes to the mmap region, we
close the fd to free up the slot in the kernel table for Parrot.

Fixes #431.
Check fd is writable.
Allow Parrot to close unneeded mmap fd.

* parrot-mmap-fd:
  Check fd is writable.
  Allow Parrot to close unneeded mmap fd.
  whitespace
  Add test for mmap using Parrot fd unnecessarily.