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.result

@@ -210,6 +210,7 @@ select HEX(f) from t4;
 HEX(f)
 835C
 flush logs;
+flush logs;
 select * from t5  /* must be (1),(1) */;
 a
 1
@@ -250,3 +251,4 @@ call p1();
 1
 drop procedure p1;
 drop table t1, t2, t03, t04, t3, t4, t5;
+flush logs;