gprs: Add TLV parse functions

This adds a set of function that parse a single tlv, lv, tv, or v
encoded information element. They are complementary to the *_put
functions defined in libosmocore's tlv.h file. The functions update
the data and data_len fields unless they are a 'match' function and
the tag field doesn't match.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index 5f2d81e..f99087d 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -1,4 +1,5 @@
 ===== GbProxy test START
+Test shift functions
 === test_gbproxy ===
 --- Initialise SGSN ---