This is very similar to the existing mbstring.regex_stack_limit,
but for backtracking. The default value matches pcre.backtrack_limit.
Only used on libonig >= 2.8.0.

Add a max_depth option to unserialize and an unserialize_max_depth
ini setting, which can be used to control the depth limit. The
default value is 4096.

This option is intended to prevent stack overflows during the
unserialization of deeply nested structures.

This fixes bug #78549 and addresses oss-fuzz #17581, #17589, #17664,
and #17788.

Formerly, there was at most a single OPcache instance per user and the
so called system ID (which is determined from the PHP version).
Sometimes multiple OPcaches might be desired, though, particularly for
unrelated CLI scripts, which may even be necessary (e.g. for our test
suite in parallel mode).

We therefore introduce a new INI directive `opcache.cache_id` which
allows to configure independent OPcache instances for the same user.

We also use `GetUserNameW()` instead of `php_win32_get_username()`,
because the latter retrieves the user name encoded in the
`default_charset`, which can obviously yield different results for
different charsets, leading to OPcache "incompatibilities".  Slightly
worse, some characters may not even be encodeable in the
`default_charset` and would be replaced by question marks, which could
result in different users sharing the same OPcache.

We also refactor, and re-use existing APIs to avoid duplicated code.