mysql RA improvements

This patch

* fixes an error that comes up when replication is not working: `$secs_behind` is `NULL`, which causes an error in numerical comparisons and leads `if` to choose the wrong branch (thus leaving the `reader` attribute on)
* <del>adds a `writable` attribute for the master</del>
* fixes a line indent in `check_slave` (sorry, I just _had_ to do it :-/)