make all library-internal static buffers thread-local

We have a number of library-internal static global buffers which are
mainly used for various stringification functions.  This worked as
all of the related Osmocom programs were strictly single-threaded.

Let's make those buffers at least thread-local.  This way every thread
gets their own set of buffers, and it's safe for multiple threads to
execute the same functions once.  They're of course still not
re-entrant.  If you need re-entrancy, you will need to use the _c()
or _buf() suffix version of those functions and work with your own
(stack or heap) buffers.

Change-Id: I50eb2436a7c1261d79a9d2955584dce92780ca07
diff --git a/src/gsm/abis_nm.c b/src/gsm/abis_nm.c
index a4c0e41..3fb8f0f 100644
--- a/src/gsm/abis_nm.c
+++ b/src/gsm/abis_nm.c
@@ -939,7 +939,7 @@
 
 const char *abis_nm_dump_foh(const struct abis_om_fom_hdr *foh)
 {
-	static char foh_buf[128];
+	static __thread char foh_buf[128];
 	return abis_nm_dump_foh_buf(foh_buf, sizeof(foh_buf), foh);
 }