WL#5142 FLUSH LOGS should take optional arguments for which log(s) to flush

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.

sql/log.cc

@@ -965,6 +965,54 @@ bool LOGGER::flush_logs(THD *thd)
 }
 
 
+/**
+  Close and reopen the slow log (with locks).
+  
+  @returns FALSE.
+*/
+bool LOGGER::flush_slow_log()
+{
+  /*
+    Now we lock logger, as nobody should be able to use logging routines while
+    log tables are closed
+  */
+  logger.lock_exclusive();
+
+  /* Reopen slow log file */
+  if (opt_slow_log)
+    file_log_handler->get_mysql_slow_log()->reopen_file();
+
+  /* End of log flush */
+  logger.unlock();
+
+  return 0;
+}
+
+
+/**
+  Close and reopen the general log (with locks).
+
+  @returns FALSE.
+*/
+bool LOGGER::flush_general_log()
+{
+  /*
+    Now we lock logger, as nobody should be able to use logging routines while
+    log tables are closed
+  */
+  logger.lock_exclusive();
+
+  /* Reopen general log file */
+  if (opt_log)
+    file_log_handler->get_mysql_log()->reopen_file();
+
+  /* End of log flush */
+  logger.unlock();
+
+  return 0;
+}
+
+
 /*
   Log slow query with all enabled log event handlers