Mysql check improvements

[LotharSee]

I already did some changes in the code of MySQL check since 3.8.0 (bugfixes: 68975c5, ddc56aa, refactoring: 5517c79), they should be part of this PR but they are already on master.

This PR closes #575, it adds port as an configuration parameter. It shouldn't be a problem.

It also provides a solution for #442 / #448. It concerns system metrics for MySQL, the issue is well described in the previous 2 tickets.
Instead of adding a parameter in the configuration to collect these metrics, I collect them only if I am sure they will be right:

• Server connection on UNIX socket / localhost : be sure that the server is running locally
• Get pid from ps only if it returns only 1 result, avoid error in case of multiple server instances
• Get pid from ps only on linux: no way to filter precisely on FreeBSD and Darwin
• An exception during the opening of the /proc/ file (no permission, file doesn't exist) isn't an error
• Don't display warning if system metrics aren't collected

That's a proposal. @clutchski, @remh, tell me what you think about it.