fpm: relax log permissions (bug #61435)

Do not unconditionally set permissions for access_log, error_log and slow_log to
0600, instead default to 0644 and allow the administrator to set tightier
permissions as needed (using umask).

This makes the logs more accessible to developers which are unlikely to run as
the PHP server user.
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[bug 61435][1] has been open for too long, let's try to carry it upstream in this way.

 [1]: https://bugs.php.net/bug.php?id=61435