Implementing this feature connected to wl#3368 mixed binlog_format default.
  Supplied by my.cnf or explicitly in command line option gets be displayed.
  When not supplied `(No default value)' is displayed, even though --log-bin might
  be supplied. The option is different object from @@global.binlog_format variable.
  The default `mixed' for the latter is dependant on presence of `--log-bin' option,
  otherwise the value of the var is set to NULL (undefined):

   var := opt | MIXED  when binlog-in-use
   var := NULL         otherwise (no binlog, no format)

  Comments on NDB and mixed format updated, also dependency the option on --log-bin
  aka binlog-in-use is worded.
  
  Making t/rpl_switch_stm_row_mixed.test to interprete DEFAULT for binlog_format
  as MIXED.
  Comments on what the test covers are added.

  todo/fixme: turning @@global.binlog_format to be read-only when it's set to NULL (no binlog).
  todo/fixme: options dependacy (acyclic) graph, particularly to solve a task of
setting defaults values for the leaf nodes
  only when parents' nodes are set.