move creation of insert subscriber data messages to a common function

Move code to create an Insert Subscriber Data message into a common
function which can be shared by hlr.c and luop.c.

As a consequence, we always encode gsup.cn_domain in the corresponding
msgb and must adjust expected output of the 'gsup' test accordingly.

Change-Id: I6a92ca34cdaadca9eacc774bb1ca386c325ba865
Requested-by: neels
Related: OS#2785
5 files changed