Fix --keep-old-files option.

The regression was introduced by 8f390db9.  This patch implements additional
option --skip-old-files, which silently skips members which would cause
writing over existing files, and restores --keep-old-files to its traditional
behavior.

* NEWS: Update.
* configure.ac: Update.
* doc/tar.texi: Document the changes.
* src/common.h (SKIP_OLD_FILES): New old_files mode.
* src/extract.c (maybe_recoverable): Restore KEEP_OLD_FILES behavior.
Handle SKIP_OLD_FILES.
* src/tar.c: New option --skip-old-files.
* tests/extrac18.at: New file.
* tests/extrac19.at: New file.
* tests/Makefile.am: Add new test cases.
* tests/testsuite.at: Likewise.