level fixes

fix incorrect handling of INFO*
allow logging to be disabled via NONE or DISABLED
more efficient enabled/disabled checking
test case-insensitivity
fix doc typos + test warning

\* INFO is borked (it's higher than ERROR) e.g. setting LOG_DECLARE_STARTUP_LEVEL=INFO **disables WARN and ERROR** and setting LOG_DECLARE_STARTUP_LEVEL=WARN enables INFO. This makes it a pretty much impossible to filter out logspam.