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/testsuite.at b/tests/testsuite.at
index a542798..85c3e8b 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -84,6 +84,12 @@
 AT_CHECK([$abs_top_builddir/tests/gsm0408/gsm0408_test], [0], [expout], [ignore])
 AT_CLEANUP
 
+AT_SETUP([gprs])
+AT_KEYWORDS([gprs])
+cat $abs_srcdir/gprs/gprs_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/gprs/gprs_test], [0], [expout], [ignore])
+AT_CLEANUP
+
 AT_SETUP([logging])
 AT_KEYWORDS([logging])
 cat $abs_srcdir/logging/logging_test.ok > expout