Fix `--test-label' and `--label -r' behavior.

* doc/tar.texi (Including a Label in the Archive): Revise
the section.
* NEWS: Update

* src/buffer.c (open_archive): Check volume label on
ACCESS_UPDATE as well.
* src/list.c (test_archive_label): Rewrite to match the
documentation.
* src/names.c (regex_usage_warning): Return int.
(names_notfound): Rewrite the conditional.
(label_notfound): New function.

* tests/label03.at: New testcase.
* tests/label04.at: New testcase.
* tests/label05.at: New testcase.
* tests/Makefile.am: Add new testcases.
* tests/testsuite.at: Likewise.