Fix GH-9139: Allow FFI in opcache.preload when opcache.preload_user=root

[arnaud-lb]

opcache.preload can not be used for FFI preloading when running as root, because of a chain of causes:

• opcache.preload requires opcache.preload_user when running as root
• FFI does not work when opcache.preload_user is defined, because the preloading happens in a sub-process in this case

It is not strictly necessary to preload in a sub-process when opcache.preload_user is the current user, because we are not going to change the uid of the process.

This PR skips the fork() when opcache.preload_user is the current user, thus allowing FFI in opcache.preload when running as root, as long as opcache.preload_user=root.

Although running as root is not recommended, it is common practice with docker containers. This change also allows to have at least one CI pipeline running as root.

Fixes #9139

Most changes are whitespaces.

[iluuu1994]

The implementation looks good, although I don't know enough about this code to understand if there are any implications here. Note that the documentation will need to be adjusted.

https://www.php.net/manual/en/opcache.preloading.php

-Running preloading as root is not allowed.
+Running preloading as root is disallowed by default. Set `opcache.preload_user=root` to explicitly allow it.

Or something along those lines.