Add macro for checking if some flags are set

This way we don't need to repeat the argument twice.
I didn't replace all instances. I think it's better to leave out:
- asserts
- comparisons like x & y == x, which are mathematically equivalent, but
  here we aren't checking if flags are set, but if the argument fits in the
  flags.

src/basic/capability-util.h

@@ -41,7 +41,7 @@ static inline void cap_free_charpp(char **p) {
 static inline bool cap_test_all(uint64_t caps) {
         uint64_t m;
         m = (UINT64_C(1) << (cap_last_cap() + 1)) - 1;
-        return (caps & m) == m;
+        return FLAGS_SET(caps, m);
 }
 
 bool ambient_capabilities_supported(void);