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.


client/mysqlbinlog.cc:
  Implementing --force-if-open option with TRUE by default
mysql-test/mysql-test-run.pl:
  Using --disable-force-if-open for all tests to avoid
  sporadic test failures because of running "mysqlbinlog"
  on a non-flushed binlog files.
mysql-test/r/binlog_row_mix_innodb_myisam.result:
  FLush log before running dumping.
mysql-test/r/binlog_stm_mix_innodb_myisam.result:
  FLush log before running dumping.
mysql-test/r/mysqlbinlog.result:
  FLush log before running dumping.
mysql-test/r/mysqlbinlog2.result:
  FLush log before running dumping.
mysql-test/r/mysqlbinlog_base64.result:
  FLush log before running dumping.
mysql-test/r/user_var-binlog.result:
  FLush log before running dumping.
mysql-test/t/binlog_row_mix_innodb_myisam.test:
  FLush log before running dumping.
mysql-test/t/binlog_stm_mix_innodb_myisam.test:
  FLush log before running dumping.
mysql-test/t/mysqlbinlog.test:
  FLush log before running dumping.
  
  Adding new tests:
  - checking that $MYSQL_BINLOG returns an error on a non-closed binlog
  file because of --disable-force-if-open
  - checking that it does not return an error with --force-if-open
mysql-test/t/mysqlbinlog2.test:
  FLush log before running dumping.
mysql-test/t/mysqlbinlog_base64.test:
  FLush log before running dumping.
mysql-test/t/user_var-binlog.test:
  FLush log before running dumping.