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.

minidlna.conf

@@ -72,3 +72,7 @@ model_number=1
 
 # always force SortCriteria to this value, regardless of the SortCriteria passed by the client
 #force_sort_criteria=+upnp:class,+upnp:originalTrackNumber,+dc:title
+
+# maximum number of simultaneous connections
+# note: many clients open several simultaneous connections while streaming
+#max_connections=50