Implement the --warning option.

Implement the --warning option.

* src/warning.c: New file.
* src/Makefile.am: Add warning.c
* src/common.h: Provide definitions for warning classes.
(warning_option): New global.
(WARNOPT): New define.
(set_warning_option): New prototype.
* src/tar.c: New option `--warning'.
* src/compare.c: When applicable WARNOPT instead of WARN.
* src/create.c: Likewise.
* src/extract.c: Likewise.
* src/incremen.c: Likewise.
* src/list.c: Likewise.

* NEWS, doc/tar.texi: Update.