Remove use of non-threadsafe strerror

Add helper class to use strerror_r or strerror_s

src/string_func.h

@@ -306,4 +306,17 @@ inline bool IsWhitespace(char32_t c)
 char *strcasestr(const char *haystack, const char *needle);
 #endif /* strcasestr is available */
 
+/**
+ * The use of a struct is so that when used as an argument to seprintf/etc, the buffer lives
+ * on the stack with a lifetime which lasts until the end of the statement.
+ * This avoids using a static buffer which is thread-unsafe, or needing to call malloc, which would then nee to be freed.
+ */
+struct StrErrorDumper {
+	const char *Get(int errornum);
+	const char *GetLast();
+
+private:
+	char buf[128];
+};
+
 #endif /* STRING_FUNC_H */