[RSL] split rsl_rll_push_l3() L3 LV pushign and rsl_rll_push_hdr()

This allows us to generate RSLms messages that have some non-standard
IEs between the RSL/RLL common header and the L3 INFO IE.
diff --git a/include/osmocore/protocol/gsm_08_58.h b/include/osmocore/protocol/gsm_08_58.h
index 7dc3569..5fe332e 100644
--- a/include/osmocore/protocol/gsm_08_58.h
+++ b/include/osmocore/protocol/gsm_08_58.h
@@ -49,6 +49,14 @@
 	uint8_t	data[0];
 } __attribute__ ((packed));
 
+/* Chapter 8.5 */
+struct abis_rsl_cchan_hdr {
+	struct abis_rsl_common_hdr c;
+	uint8_t	ie_chan;
+	uint8_t	chan_nr;
+	uint8_t	data[0];
+} __attribute__ ((packed));
+
 
 /* Chapter 9.1 */
 #define ABIS_RSL_MDISC_RLL		0x02
@@ -88,6 +96,7 @@
 	RSL_MT_PAGING_CMD,
 	RSL_MT_IMMEDIATE_ASSIGN_CMD,
 	RSL_MT_SMS_BC_REQ,
+	RSL_MT_CHAN_CONF,		/* non-standard element */
 	/* empty */
 	RSL_MT_RF_RES_IND			= 0x19,
 	RSL_MT_SACCH_FILL,