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.

This passes through syslog message unchanged, without splitting
messages at newlines.

Some default opcache settings were missing.

* PHP-7.3:
  Update NEWS & UPGRADING
  Add fallbacks for older oniguruma versions
  Add mbstring.regex_stack_limit to php.ini-*
  Implement RF bug #72777 - ensure stack limits on mbstring functions.