libosmogsm: (re)introduce gsm48_push_l3hdr()

There was gsm0480_l3hdr_push() declared in a header file, but
not exposed in 'libosmogsm.map'. Furthermore, for some reason
it was a part of GSM 04.80 API, what is not actually correct.

Let's rename this symbol, and properly expose it as a part of
the GSM 04.08 API. Also, let's introduce an auxiliary wrapper
for messages, where the transaction identifier is required
(see GSM 04.07, section 11.2.3.1.2).

Change-Id: I8a045efe8335d83fcbe8d43eb180972e3b1d9dda
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 8ccfa49..64833d2 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,6 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what			description / commit summary line
+libosmogsm	gsm0480_l3hdr_push()	removed from gsm/gsm0480.h (was not exposed)
+libosmogsm	gsm48_push_l3hdr()	(re)introduced in gsm/gsm48.h (GSM 04.08 API)
+libosmogsm	gsm48_push_l3hdr_tid()	a wrapper around gsm48_push_l3hdr()