gsm: Add APN conversion functions

These functions are currently part of openbsc but also needed by
other projects.

The function have been renamed as follows:

  gprs_apn_to_str -> osmo_apn_to_str
  gprs_str_to_apn -> osmo_apn_from_str

Sponsored-by: On-Waves ehf
diff --git a/tests/gprs/gprs_test.ok b/tests/gprs/gprs_test.ok
new file mode 100644
index 0000000..619c561
--- /dev/null
+++ b/tests/gprs/gprs_test.ok
@@ -0,0 +1 @@
+Done.