Properly handle unexpected non-directories.

In DIRECTORY-ENTRIES, DELETE-DIRECTORY, and DELETE-DIRECTORY-TREE,
carefully check for non-directory arguments and react accordingly.

This is related to issue #109.
Handle pax extended headers in minitar.

http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html

Paths that are too long for standard tar data structures are stuffed
into a pax extended header. This change teaches minitar how to properly
handle them.

Fixes #109.