Fix known PHP 8 issues

[jackpoz]

Fix PHP 8 issues spotted so far:

• $_SERVER['QUERY_STRING'] can be null on Windows (this happens with PHP 7.4 too)
• is_callable now returns false for instance methods when passing a static type instead of an instance of the type, see https://php.watch/versions/8.0/non-static-static-call-fatal-error
• "Cannot access offset of type string on string" in smartAI.class.php when loading /?npc=68 , caused by using selectCol instead of select and trying to access the result (which is only the value of the first column) as a row

Should fix #298 until more issues are reported

[Sarjuuk]

cherry picked the individual commits because badefc413b550d83e77a2292cb9a094cff368cc2 is wrong (i think)

referenced the issue in f9ed75d5afeecd507a92aecfc2baebdd21c5e5d4