mkfs -a understands how to quote filenames with %q but
mkext doesn't know how to unpack them.

My fix doesn't allow any of the fields in the header to be
zero length, however looking at mkfs it uses %q for the filename
and uid and gid so I don't beleive this is a problem.

Perhaps there some old mkfs archives which have missing uid or gid
that this change might break?

-Steve