Detect tarbombs while extracting

Detect tarbombs while extracting

* src/common.h (one_top_level_option): New global.
(one_top_level): New global.
* src/extract.c (extr_init): If one_top_level_option is set, determine
the name one_top_level that might have to be prepended.
(extract_archive): If one_top_level_option is set, prepend one_top_level
to all names that don't already start with it.
* src/tar.c (ONE_TOP_LEVEL_OPTION): New contant.
(options): New option --one-top-level.
(parse_opt): Handle this option.
(decode_options): Make it conflict with --absolute-names.