MINOR: config: continue to rely on DEFAULT_MAXCONN to set the minimum maxconn

Some packages used to rely on DEFAULT_MAXCONN to set the default global
maxconn value to use regardless of the initial ulimit. The recent changes
made the lowest bound set to 100 so that it is compatible with almost any
environment. Now that DEFAULT_MAXCONN is not needed for anything else, we
can use it for the lowest bound set when maxconn is not configured. This
way it retains its original purpose of setting the default maxconn value
eventhough most of the time the effective value will be higher thanks to
the automatic computation based on "ulimit -n".
diff --git a/include/common/defaults.h b/include/common/defaults.h
index 80ef2c9..fbacca4 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -192,11 +192,15 @@
  * absolute limit accepted by the system. If the configuration specifies a
  * higher value, it will be capped to SYSTEM_MAXCONN and a warning will be
  * emitted. The only way to override this limit will be to set it via the
- * command-line '-n' argument.
+ * command-line '-n' argument. If SYSTEM_MAXCONN is not set, a minimum value
+ * of 100 will be used for DEFAULT_MAXCONN which almost guarantees that a
+ * process will correctly start in any situation.
  */
 #ifdef SYSTEM_MAXCONN
 #undef  DEFAULT_MAXCONN
 #define DEFAULT_MAXCONN SYSTEM_MAXCONN
+#elif !defined(DEFAULT_MAXCONN)
+#define DEFAULT_MAXCONN 100
 #endif
 
 /* Minimum check interval for spread health checks. Servers with intervals