process: make max number of children (connections) configurable

At least some Panasonic clients try to open more than 5 simultaneous
connections to the server.  If we keep the default of 5 max children,
it results in choppy playback on those clients.

Make this setting configurable, and default to 50 max connections.
Our process is pretty lightweight, so 50 children should not be a
problem on most systems.

minidlnatypes.h

@@ -48,6 +48,7 @@ struct lan_addr_s {
 struct runtime_vars_s {
 	int port;	/* HTTP Port */
 	int notify_interval;	/* seconds between SSDP announces */
+	int max_connections;	/* max number of simultaneous conenctions */
 	char *root_container;	/* root ObjectID (instead of "0") */
 	char *ifaces[MAX_LAN_ADDR];	/* list of configured network interfaces */
 };