Update Debian mpm_prefork.conf to include ServerLimit (#152)

If MaxRequestWorkers is greater than 256 than you also need to set the
ServerLimit value which is defaulted to 256 but has a 20000 hard maximum

apache/files/Debian/mpm/mpm_prefork.conf.jinja

 # prefork MPM
 # StartServers: number of server processes to start
 # MinSpareServers: minimum number of server processes which are kept spare
+# ServerLimit: maximum number of server processes
 # MaxSpareServers: maximum number of server processes which are kept spare
 # MaxRequestWorkers: maximum number of server processes allowed to start
 # MaxConnectionsPerChild: maximum number of requests a server process serves
 <IfModule mpm_prefork_module>
 	StartServers {{ mpm_param['start_servers'] | d('5') }}
 	MaxRequestWorkers {{ mpm_param['max_request_workers'] | d('150') }}
+{%- if mpm_param['max_request_workers'] | d('150') >= 256 %}
+	ServerLimit {{ mpm_param['max_request_workers'] | d('150') }}
+{%- endif %}
 	MinSpareServers {{ mpm_param['min_spare_servers'] | d('5') }}
 	MaxSpareServers {{ mpm_param['max_spare_servers'] | d('10') }}
 	MaxConnectionsPerChild {{ mpm_param['max_connections_per_child'] | d('0') }}