Introduce instance_name field into xrdp.ini and xrdp-sesrun, along with

the N policy in sesman.ini, allowing xrdp sessions to be tagged with an
instance name to enable persistent association with a specific
xrdp instance, to allow experiences where users reconnect to specific
sessions based on e.g. the xrdp listening port used.

common/xrdp_constants.h

@@ -43,6 +43,12 @@
  */
 #define MAX_PEER_ADDRSTRLEN 46
 
+/**
+ * Max length of buffer containing an instance name used to
+ * tag sessions for discrimination on reconnection.
+ **/
+#define MAX_XRDP_INSTANCE_NAMELEN 80
+
 /**
  * Size of buffer including terminator for a socket description, as
  * returned by g_sck_get_peer_description()