lapdm: Append RSL_IE_OSMO_ABS_FRAME_NUMBER to RSLms msgs towards upper layers

This makes it possible to track GSM time in the upper layers.
The existing RSL_IE_FRAME_NUMBER and RSL_IE_STARTNG_TIME cannot be used
there, since those are 16bit fields containing Relative FN values.

The IE needs to be added before the L3_INFO one, because user code
usually assumes the msgb->l3 pointing to L3_INFO value extends until the
end of the message, using msgb3_len(msg). Regarding having an extra IE
at the middle, it's not a big problem since the libosmocore version
submitting this commit to the upper layers is the same which will also
be parsing it through rsl_tlv_parse() later on by the app.

Related: OS#3626
Change-Id: Id62c18f49f270449067b25b7104eb8b47f1955ec
---
M include/osmocom/gsm/rsl.h
M src/gsm/lapdm.c
M src/gsm/rsl.c
M tests/lapd/lapd_test.c
M tests/lapd/lapd_test.ok
5 files changed, 74 insertions(+), 13 deletions(-)