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
Not all extensions consistently throw exceptions when the user passes
a path name containing null bytes. Also, some extensions would throw
a ValueError while others would throw a TypeError. Error messages
also varied.
Now a ValueError is thrown after all failed path checks, at least for
as far as these occur in functions that are exposed to userland.
Closes GH-6216.
