dist: change tar format default to ustar.

Suggested in https://bugs.gnu.org74847.

* m4/init.m4 (tar-ustar): make ustar the default instead of tar-v7.
* doc/automake.texi (tar-formats): document this.
* NEWS: mention this.
* t/tar-override.sh: force tar-v7 for purposes of this test.
4 files changed