gh-144001: Simplify Base64 decoding with altchars and ignorechars specified (GH-144324)

Treat "+" and "/" like other characters not in the alternative Base64
alphabet when both altchars and ignorechars are specified.
E.g. discard them if they are not in altchars but are in ignorechars,
and set error if they are not in altchars and not in ignorechars.
Only emit warnings if ignorechars is not specified.