cap-list: add CAPABILITY_TO_STRING() macro using compound initialization to allocate fallback buffer

Let's add a helper that can return a numeric string in case we don't
recognize a name for a capability.

src/basic/cap-list.h

@@ -3,7 +3,15 @@
 
 #include <inttypes.h>
 
+/* Space for capability_to_string() in case we write out a numeric capability because we don't know the name
+ * for it. "0x3e" is the largest string we might output, in both sensese of the word "largest": two chars for
+ * "0x", two bytes for the hex value, and one trailing NUL byte. */
+#define CAPABILITY_TO_STRING_MAX (2 + 2 + 1)
+
 const char *capability_to_name(int id);
+const char *capability_to_string(int id, char buf[static CAPABILITY_TO_STRING_MAX]);
+#define CAPABILITY_TO_STRING(id) capability_to_string(id, (char[CAPABILITY_TO_STRING_MAX]) {})
+
 int capability_from_name(const char *name);
 int capability_list_length(void);