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.