Fix handling of directory entries

Don't emit tar blocks for directories with an invalid nonzero size.  Also, if
such an entry is encountered during restore, don't actually attempt to treat
it as valid and thus skip over the next actual tar entry.

This patch also adds tracking of the data actually consumed during restore,
and reports a total at the end of stream.

Change-Id: I625173f76df3c007e899209101ff2b587841f184