Custom GUCs and typos

1) On top of OP's patch I added support to warn if the prefix of custom GUC is invalid,for valid questions such as "How do you know that's a bogus prefix?  It could perfectly well be a fully valid setting for an extension that the installation doesn't choose to preload.",we can get the info of such extensions using extension_file_exists() which tells that its installed but not preloaded thanks to Greg for proposing this,which tells this could be a potential valid extension ,so if its not it in reserved_class_prefix and also not in prefix/share/extension dir then the prefix is for sure invalid,so i warn and remove the GUC from hashtable.

2) for preloaded extensions if the suffix/parameter is a typo or if we want to create a new custom GUC for ex:"pg_stat_statements.count_something = 1", currently we throwing a warning as

WARNING:  invalid configuration parameter name "pg_stat_statements.count_something", removing it