boot: suppress XBOOTLDR if same device as ESP when enumerating entries

On my local system I linked up the ESP and XBOOTLDR partitions, and
ended up with duplicate entries being listed. Try hard to detect that
and only enumerate entries in the ESP if it turns out that both dirs
have the same dev_t.

This should detect both bind mounted and symlinked cases and should make
our list output less confusing.

src/basic/stat-util.h

@@ -114,3 +114,9 @@ int statx_fallback(int dfd, const char *path, int flags, unsigned mask, struct s
                 struct new_statx nsx;           \
         } var
 #endif
+
+static inline bool devid_set_and_equal(dev_t a, dev_t b) {
+        /* Returns true if a and b definitely refer to the same device. If either is zero, this means "don't
+         * know" and we'll return false */
+        return a == b && a != 0;
+}