Should have a mechanism to disable log censoring of the USER/USERNAME environment variable

[irydacea]

Describe the desired feature

Wesnoth 1.18 forcefully censors every single log line via lg::sanitize_log(), resulting in any instances of the player's USER (POSIX) or USERNAME (Windows) environment variable contents surrounded in / or \ (*) being replaced with /USER/ or \USER\. This can be rather unhelpful in situations where a dev is trying to gather diagnostic information, or when piping Wesnoth's output to another process, so ideally there should be a --no-log-sanitize switch to disable this feature.

As far as I can tell, the candidates for places to plug a check for this switch into are lg::sanitize_log(), or lg::log_in_progress::operator|() around the call to sanitize_log(), if there is a desire to otherwise preserve the latter's functionality for some other purpose despite the switch being in use.

(*) Incidentally, this will not work in the event that the name is surrounded by a backslash and a forward slash, or viceversa — as in two different types of slash rather than two matching ones.