Add 'readdir' call.

Add 'readdir' call.

This adds a readdir call (mostly intended for programs).  The
return value is a list of guestfs_dirent structures.

This adds the new types 'struct guestfs_dirent' and
'struct guestfs_dirent_list', along with all the code to
return these in the different language bindings.

Also includes additional tests for OCaml and Perl bindings
to test this.