Restore conversion to lower case in G_SanitiseString(), with a few caveats.

This can't sensibly deal with certain languages having different
capitalisation, e.g. German ‘SS’/‘SZ’⇔‘ß’ and Turkish ‘i’⇔‘İ’ and ‘ı’⇔‘I’ –
we don't have the context for this without doing things like using clients'
locale information here. (Though German ‘ẞ’⇔‘ß’ works.)