gcov-io.h: Allow zero tag as EOF indicator.

	* gcov-io.h: Allow zero tag as EOF indicator.
	(gcov_is_eof): Remove.
	(gcov_truncate): Remove.
	* gcov-io.c (gcov_seek): Always SEEK_SET.
	* libgcov.c (gcov_exit): Don't use gcov_truncate.  Write explicit
	EOF tag.
	* coverage.c (read_counts_file): Don't use gcov_is_eof.
	* gcov-dump.c (dump_file): Likewise.
	* gcov.c (read_graph_file, read_count_file): Likewise.

From-SVN: r85291