Added support for warning about and skipping files.

Files that do not exist or user does not have read access to
are skipped and users are warned that the file is skipped.
Enabled skipping and warning special files.

Files include character special devices, block special devices,
FIFO, and sockets.
Added Windows compatibility to warning about files.
Merge pull request #881 from kyleknap/warnings

Added support for warning about and skipping files.