Fix handling of DSN extra arguments

[nabadger]

I believe this addresses the following issues

#458 #303 #376

DATA_SOURCE_NAME="my-user:my-password-with&?-chars@(127.0.0.1:3306)/ ./mysqld_exporter

This fix will just look for ? by checking only the chars that appear after the final / .

If there is no / in the DSN, then it's already being treated as an invalid DSN

invalid DSN: missing the slash separating the database name

[SuperQ]

I've been thinking about completely dropping the DSN env var parsing. It's difficult to get right.

I would suggest looking at #306, I think we would be better off eliminating this feature entirely.

[nabadger]

Makes sense - we are actually writing the .my.cnf file, it's just that the exporter takes the contents and parses them out into a DSN.

[SuperQ]

The whole DSN thing in the Go mysql driver is not fun.

[SuperQ]

We've reworked the exporter to use mysql.Config() to configure the DSN. The old DATA_SOURCE_NAME env var has been removed.