WL#1450: Flushing logs of different types separately

Currently we have "FLUSH LOGS" which flushes all log files, which is a bit raw 
and 
not convenient in some cases. We shall have command which would allow you to
flush only some particular log file.

It is especially annoying for log-error.    Having flushed your replication logs
couple of times for example for backup creation needs you result in loosing all
your previous error log. 


Also, if we allow you to only specify some, then we should add an IGNORE too, 
so that we can simply ignore one or the other.

(http://bugs.mysql.com/bug.php?id=14104)


This should also be extended to the --flush-logs option in mysqldump and the 
mysqladmin flush option as well.