Use constants for the defaults of disableipv4/disableipv6 so the code and docs cannot be out of sync

src/net.h

@@ -85,6 +85,12 @@ static const unsigned int MAX_PROTOCOL_MESSAGE_LENGTH = (_MAX_BLOCK_SIZE + 24);
 static const unsigned int MAX_SUBVERSION_LENGTH = 256;
 /** -listen default */
 static const bool DEFAULT_LISTEN = true;
+
+/** -disableipv4 default */
+static const bool DEFAULT_DISABLE_IPV4 = false;
+/** -disableipv6 default */
+static const bool DEFAULT_DISABLE_IPV6 = false;
+
 /** The maximum number of entries in mapAskFor */
 static const size_t MAPASKFOR_MAX_SZ = MAX_INV_SZ;
 /** The maximum number of entries in setAskFor (larger due to getdata latency)*/