During code review, it was indicated that some TLV protocols that we
will likely deal with in the near future also employ an I, and instance
value of a tag. Add TLIV support.
A usage example for a manually implemented TLIV structure is found in
tests/libosmo-gtlv/gtlv_test.c.
A usage example for a generated TLIV protocol is found in
tests/libosmo-gtlv/test_tliv/.
Related: SYS#5599
Change-Id: I0a076e54dfba6038cc779cb7c8f3967d212226aa
