Convert log_warnings from numeric level to feature mask

When checking

https://mariadb.com/kb/en/server-system-variables/#log_warnings

I see a total of 28 different things that can be logged as warning across the different log_warning levels.

Sometimes you might be interested in warnings of higher levels, like e.g.

"Connections aborted due to "Too many connections" errors. " (level 4)

but not in lower level warnings. Especially

"Aborted connection ... (Got an error reading communication packets)" (level 2)

comes to mind, as some installations have clients that just disconnect without prior mysql_close() and can't do anything about it, e.g. when not having access to application source code.

So error log can be spammed with "Aborted connection ..." messages, making it hard to check for actual problems, but at the same time lowering the log_warning level to 1 to get rid of "Aborted connection ..." log lines would also hide other warnings completely.

Given the amount of different warning types covered by log_warnings logic by now, it seems to be about time to convert log_warnings into a feature list / mask instead, where individual options can be set and removed, like we e.g. have it with sql_mode and optimizer_switch settings.

The current warning level numbers could still be supported in such a scheme, by making them shortcuts that combine several options, similar to e.g. the TRADITIONAL option in sql_mode ...