For rationale, see https://github.com/php/php-src/pull/6787
Make extension checks lowercase, add a special case for opcache
that has internal name not matching .so filename.
Extensions migrated in part 2:
* dom
* exif
* fileinfo
* ffi
According to the DOM specification, this argument should be
nullable. It's also supposed to be a required argument, but
not changing that at this point.
