Add _buf() functions to bypass static string buffers

We have a number of static buffers in use in libosmo*.  This means
the related functions are not usable in a thread-safe way.  While
we so far don't have many multi-threaded programs in the osmocom
universe, the static buffers also prevent us from calling the same
e.g. string-ify function twice within a single printf() call.

Let's make sure there's an alternative function in all those cases,
where the user can pass in a caller-allocated buffer + size, and make
the 'classic' function with the static buffer a wrapper around that
_buf() variant.

Change-Id: Ibf85f79e93244f53b2684ff6f1095c5b41203e05
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index f8f72a7..f48cc68 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -180,6 +180,7 @@
 
 /* Returns static buffer with string representation of a GSM Time */
 char *osmo_dump_gsmtime(const struct gsm_time *tm);
+char *osmo_dump_gsmtime_buf(char *buf, size_t buf_len, const struct gsm_time *tm);
 
 /* GSM TS 03.03 Chapter 2.6 */
 enum gprs_tlli_type {