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/core/msgb.h b/include/osmocom/core/msgb.h
index 5029225..0c51ce2 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -67,6 +67,7 @@
 extern void msgb_reset(struct msgb *m);
 uint16_t msgb_length(const struct msgb *msg);
 extern const char *msgb_hexdump(const struct msgb *msg);
+char *msgb_hexdump_buf(char *buf, size_t buf_len, const struct msgb *msg);
 extern int msgb_resize_area(struct msgb *msg, uint8_t *area,
 	int old_size, int new_size);
 extern struct msgb *msgb_copy(const struct msgb *msg, const char *name);