Allow configuration of expire-logs-days

[geerlingguy]

I just noticed that, on one of my super-heavy-write DB servers, the disk was almost out of space, and a little digging found that there were hundreds of 101MB mysql-bin.000XXX log files in /var/lib/mysql. While it's important to keep logs for replication, you don't need all the logs to the beginning of time!

I'd like to add configuration for the expire-logs-days setting, with a sane default.

[geerlingguy]

Looks like I already had the config variable hardcoded at 10 days. I'm switching it to a user-supplied variable with a default of 10.