Other drivers (including the cache) never supported fetch modes that are
not listed in `FetchMode`. Converting between the contants is only done
for PDO, so BC only needs to be restored here. PDO will validate the
fetch mode, we don’t have to.

A string is made of characters, an array might contain anything.

array_merge will not work with any iterable

The docs only talk about 2008 and up, not sure if those should be added
to 2005 too.

It was possible to provide escaping through the second argument but that
was more like a hack.

Inherit table charset/collation from db charset

Expressed PARAM_*_ARRAY through other constants to eliminate the dependency on the actual constant values