`YOURLS_DEBUG` and `yourls_debug_mode()` discrepancy

[ozh]

Functions regarding the debug mode could be less dumb

Currently, yourls_debug_mode() sets the debug mode on or off (without much validation of the var, btw) but yourls_get_debug_mode() doesn't care and only checks if the var is set and its value
(the var is ALWAYS set, by the way : https://github.com/YOURLS/YOURLS/blob/master/includes/Config/Config.php#L204-L206)

yourls_get_debug_mode() should get the actual debug mode, which could override the const value.

[ozh]

Also : the function yourls_debug_log() should probably be completely shortcut if YOURLS_DEBUG is false. Currently, it's not logging SQL queries, but do log other messages.

Also : there could be a ton of other yourls_debug_log() in the code, not just for display (in redirection, etc)