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/t/mysqlbinlog2.test

@@ -72,6 +72,7 @@ select "--- Local with 2 binlogs on command line --" as "";
 
 # This is to verify that some options apply only to first, or last binlog
 
+flush logs;
 --exec $MYSQL_BINLOG --short-form $MYSQLTEST_VARDIR/log/master-bin.000001 $MYSQLTEST_VARDIR/log/master-bin.000002
 
 --disable_query_log