GSUP: add CHECK-IMEI message

Implement necessary messages for Procedure Check_IMEI_VLR (TS 23.018
Chapter 7.1.2.9). This lets the VLR ask the EIR to check if an IMEI
is valid. In the Osmocom stack, we don't have an EIR and this request
will be handled by the HLR. We will be able to store the IMEI in the
HLR as side-effect (OS#2541).

This is roughly based on TS 29.002 8.7.1 MAP_CHECK_IMEI service, but
only implements the bare minimum required IEs (imei and imei_result).

Related: OS#3733
Change-Id: I085819df0ea7f3bfeb0cabebb5fd1942a23c6155
diff --git a/tests/gsup/gsup_test.err b/tests/gsup/gsup_test.err
index 236c38a..73b35fc 100644
--- a/tests/gsup/gsup_test.err
+++ b/tests/gsup/gsup_test.err
@@ -64,6 +64,15 @@
   generated message: 2c 01 08 21 43 65 87 09 21 43 f5 46 01 02 
   original message:  2c 01 08 21 43 65 87 09 21 43 f5 46 01 02 
   IMSI:              123456789012345
+  generated message: 30 01 08 21 43 65 87 09 21 43 f5 50 09 42 42 42 42 42 42 42 42 42 
+  original message:  30 01 08 21 43 65 87 09 21 43 f5 50 09 42 42 42 42 42 42 42 42 42 
+  IMSI:              123456789012345
+  generated message: 31 01 08 21 43 65 87 09 21 43 f5 02 01 60 
+  original message:  31 01 08 21 43 65 87 09 21 43 f5 02 01 60 
+  IMSI:              123456789012345
+  generated message: 32 01 08 21 43 65 87 09 21 43 f5 51 01 00 
+  original message:  32 01 08 21 43 65 87 09 21 43 f5 51 01 00 
+  IMSI:              123456789012345
   message 0: tested 11 truncations, 11 parse failures
   message 1: tested 14 truncations, 13 parse failures
   message 2: tested 83 truncations, 81 parse failures
@@ -86,6 +95,9 @@
   message 19: tested 20 truncations, 18 parse failures
   message 20: tested 26 truncations, 22 parse failures
   message 21: tested 14 truncations, 13 parse failures
+  message 22: tested 22 truncations, 21 parse failures
+  message 23: tested 14 truncations, 13 parse failures
+  message 24: tested 14 truncations, 13 parse failures
 DLGSUP Stopping DLGSUP logging
   message 0: tested 2816 modifications, 510 parse failures
   message 1: tested 3584 modifications, 770 parse failures
@@ -109,3 +121,6 @@
   message 19: tested 5120 modifications, 1031 parse failures
   message 20: tested 6656 modifications, 1546 parse failures
   message 21: tested 3584 modifications, 771 parse failures
+  message 22: tested 5632 modifications, 771 parse failures
+  message 23: tested 3584 modifications, 770 parse failures
+  message 24: tested 3584 modifications, 771 parse failures