ENH: add fileName::validate static method (issue #628)

- similar to word::validate to allow stripping of invalid characters
  without triggering a FatalError.

- use this validated fileName in Foam::readDir to avoid problems when
  a directory contains files with invalid characters in their names

- adjust rmDir to handle filenames with invalid characters

- fileName::equals() static method to compare strings while ignoring
  any differences that are solely due to duplicate slashes