Added fallback behavior to 'readDirStreamWithType'

 * Changed the assignment of values for missing DT_* constants. Since
   the 'd_type' field is of type 'unsigned char' we assign a negative
   number for each missing DT_* constant so they are distinguishable but
   do not collide with the values from the libc implementation.
 * Added a new 'DirStreamWithPath' that contains the path of the
   directory the directory stream belongs to.
 * 'readDirStreamWithType' falls back to a 'stat' if the 'd_type' is
   unknown or undetermined.
 * Added some tests for 'readDirStreamWithType'.