* src/names.c: tar: fix bug with --one-file-system --listed-incremental

Problem (and idea for fix) reported by Martin Weigel
<http://lists.gnu.org/archive/html/bug-tar/2010-11/msg00071.html>.
* src/common.h (is_individual_file): Remove decl.
* src/create.c (dump_file0): Replace "is_individual_file (p)"
with "top_level".
* src/incremen.c (procdir): Replace "!is_individual_file
(name_buffer)" with "st->parent".  Fix bug with --one-file-system
and --listed-incremental.
* src/names.c (individual_file_table, register_individual_file):
(is_individual_file): Remove.  All uses removed.