fixed lots of not-thread-safe things
Most of the things I fixed here were using libc functions that are
fundamentally not thread-safe.
diff --git a/aos/common/libc/aos_strsignal.h b/aos/common/libc/aos_strsignal.h
new file mode 100644
index 0000000..ef6795c
--- /dev/null
+++ b/aos/common/libc/aos_strsignal.h
@@ -0,0 +1,17 @@
+#ifndef AOS_COMMON_LIBC_AOS_STRSIGNAL_H_
+#define AOS_COMMON_LIBC_AOS_STRSIGNAL_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Thread-safe version of strsignal(3) (except it will never return NULL).
+//
+// Returns a pointer to static data or a thread-local buffer.
+const char *aos_strsignal(int signal);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // AOS_COMMON_LIBC_AOS_STRSIGNAL_H_