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.c

@@ -19,13 +19,28 @@ static const struct capability_name* lookup_capability(register const char *str,
 const char *capability_to_name(int id) {
         if (id < 0)
                 return NULL;
-
         if ((size_t) id >= ELEMENTSOF(capability_names))
                 return NULL;
 
         return capability_names[id];
 }
 
+const char *capability_to_string(int id, char buf[static CAPABILITY_TO_STRING_MAX]) {
+        const char *p;
+
+        if (id < 0)
+                return NULL;
+        if (id >= 63) /* refuse caps >= 63 since we can't store them in a uint64_t mask anymore, and still retain UINT64_MAX as marker for "unset" */
+                return NULL;
+
+        p = capability_to_name(id);
+        if (p)
+                return p;
+
+        sprintf(buf, "0x%x", (unsigned) id); /* numerical fallback */
+        return buf;
+}
+
 int capability_from_name(const char *name) {
         const struct capability_name *sc;
         int r, i;

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);