basic: introduce *_to_string_with_check() functions

They are used in later commits.

src/basic/errno-list.h

@@ -20,6 +20,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include <stdbool.h>
 /*
  * MAX_ERRNO is defined as 4095 in linux/err.h
  * We use the same value here.
@@ -28,3 +29,6 @@
 
 const char *errno_to_name(int id);
 int errno_from_name(const char *name);
+static inline bool errno_is_valid(int n) {
+        return n > 0 && n <= ERRNO_MAX;
+}