hashsplit: skip uncaching when mmap is unsupported

hashsplit: skip uncaching when mmap is unsupported

Skip our fmincore based uncaching for any file region where mmap throws
an mmap.error with errno set to ENODEV, i.e.:

  mmap.error: [Errno 19] Operation not supported by device

by having fmincore return None.  This happens on some platforms when the
input is a pipe, i.e.:

  ... | bup split ...

Thanks to Thomas Klausner and Greg Troxel for reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>