WebRequest: Use getRawVal instead of getGPCVal where possible

WebRequest: Use getRawVal instead of getGPCVal where possible

Avoid getGPCVal() for simple methods that only need a boolean,
number or specific string outcome.

This saves overhead of touching $wgContLang or UtfNormal\Validator
and makes load.php initialisation no longer depend on it.

Change-Id: I8ce1fa31f5102b3fa18b0d0b9f56c42cb90146a1