Add whitelist support to OPcache

Description

OPcache currently supports excluding files using opcache.blacklist_filename. This works well when the number of files to exclude is small.

However, in a multi-tenant application, the opposite can be true. There may be a large number of tenant-specific files that should not be cached, while only a small set of common application files should be cached.

For example, instead of maintaining:

/var/www/tenant-a/*
/var/www/tenant-b/*
/var/www/tenant-c/*
...

it would be useful to define the files that are allowed to be cached:

opcache.whitelist_filename=/path/to/whitelist

with:

/var/www/app/src/*
/var/www/app/vendor/*

The behavior would simply be the inverse of the blacklist: when a whitelist is configured, only matching files are cached.

I think blacklist and whitelist should be mutually exclusive, with PHP-FPM failing at startup if both are configured.

This would be particularly useful for applications where the set of files that should be cached is small and well-defined, while the set of files to exclude is large or dynamic.

I'm happy to implement this and open a PR if this proposal makes sense.

Start by examining OPcache's existing opcache.blacklist_filename configuration and the PHP-FPM startup behavior described in the proposal. Determine where whitelist configuration and blacklist/whitelist exclusivity belong; done means only whitelisted files are cached and PHP-FPM fails at startup when both settings are configured.