Fix race conditions with Apache vhost vs. per dir config

Fix race conditions with Apache vhost vs. per dir config

If the same php_value/php_flag is present in httpd.conf and .htaccess,
the key is potentially shared between threads. Unfortunately we can't
intern these keys using the current mechanism, because the MPM is setup
before the SAPI module setup is even started. A more elegant way were
to implement a kind of string pool for the thread safe Apache SAPI
config directives with the mechanism similar to what is done for the
SAPI setup now, but doing a separate management.