New option --sync-binlog=x (and global settable variable) which will fsync the binlog
after every x-th disk write to it. That is, if in autocommit mode, after every x-th statement
written to the binlog; if using transactions, after every x-th transaction written to the binlog.
x==0 means no fsync. x==1 is the slowest.
There is no test added for this, I have just checked that it works as --sync-binlog=1 dramatically
slows down mysqld.
Made sync-frm a global settable variable.


sql/log.cc:
  every sync_binlog_period-th disk binlog write, we fsync the binlog
sql/mysql_priv.h:
  new option sync_binlog
sql/mysqld.cc:
  new option sync_binlog
sql/set_var.cc:
  Making sync-frm a settable global option.
  New settable global option sync-binlog.
sql/set_var.h:
  new global settable variable sync_binlog needs a specific ::update because it needs to take LOCK_log