* UnixDirectoryInfo.cs: Fix Parent property so that it properly handles

	* UnixDirectoryInfo.cs: Fix Parent property so that it properly handles
	  returning the parent of "/".  UnixPath.GetDirectoryName() returns "" on
	  "error", not null; act accordingly.
	* UnixDriveInfo.cs: Fix IsReady property so that it returns `false' if no
	  media is mounted.  The presence of media is determined by comparing the
	  Statvfs.f_fsid value of the current and parent directories; if they
	  differ, it is assumed that media is present, and thus the directory is
	  ready.  If they're identical, then media isn't present, and the directory
	  isn't ready.
	* UnixPath.cs: UnixPath.GetDirectoryName("/mnt") should return "/", not "".

svn path=/trunk/mcs/; revision=91452