Assume directory members have size=0.

Assume directory members have size=0.

Since commit b8d8a61b, tar started honoring size field in headers
of the directory archive members when listing and extracting. That
doesn't seem right: although GNU tar always stores 0 for such members,
there are other tar implementation that don't. As a result, GNU tar
skips the actual directory contents when listing or extracting archives
created by such implementations.

This commit fixes that by assuming that size is 0 for directory archive
members.

* src/list.c (member_is_dir): Restore function.
(skim_member): Don't apply skim_file to directory members.
* tests/skipdir.at: Fix expectations.