Support for flushing individual logs, so that the user can
selectively flush a subset of the server logs.

Flush of individual logs is done according to the 
following syntax:

  FLUSH <log_category> LOGS;

The syntax is extended so that the user is able to flush a
subset of logs:

  FLUSH [log_category LOGS,];

where log_category is one of:
  SLOW
  ERROR
  BINARY
  ENGINE
  GENERAL
  RELAY.


mysql-test/suite/rpl/r/rpl_flush_logs.result:
  Test result for WL#5142.
mysql-test/suite/rpl/t/rpl_flush_logs.test:
  Added the test file to verify if the 'flush individual log' 
  statement works fine.
sql/log.cc:
  Added the two functions to flush slow and general log.
sql/sql_parse.cc:
  Added code to flush specified logs against the option.
sql/sql_yacc.yy:
  Added code to parse the 'flush * log' statement syntax and 
  set its option to Lex->type.