Bug#17642 mysqlbinlog: Restore from row-based binlog fails

Problem: mysqlbinlog_base64 failed sporadically.

Reason: Missing "flush logs" before running $MYSQL_BINLOG,
which could start dumping the log file before server
has finished writting into it.
Fix:
- implementing --force-if-open option to "mysqlbinlog"
- adding --disable-force-if-open to make $MYSQL_BINLOG
  fail on non-closed log files, to garantee that nobody
  will forget "flush logs" in the future.
- adding "flush logs" into all affected tests.

mysql-test/r/mysqlbinlog_base64.result

@@ -6,6 +6,7 @@ update t1 set a=a+2 where a=2;
 update t1 set a=a+2 where a=3;
 create table t2 (word varchar(20));
 load data infile '../std_data_ln/words.dat' into table t2;
+flush logs;
 drop table t1;
 drop table t2;
 select * from t1;