lldp: simplify compare_func, using ?: to chain comparisons

The ?: operator is very useful for chaining comparison functions
(strcmp, memcmp, CMP), since its behavior is to return the result
of the comparison function call if non-zero, or continue evaluating
the chain of comparison functions.

This simplifies the code in that using a temporary `r` variable
to store the function results is no longer necessary and the checks
for non-zero to return are no longer needed either, resulting in a
typical three-fold reduction to the number of lines in the code.

Introduce a new memcmp_nn() to compare two memory buffers in
lexicographic order, taking length in consideration.

Tested: $ ninja -C build/ test

All test cases pass. In particular, test_multiple_neighbors_sorted()
in test-lldp would catch regressions introduced by this commit.

src/basic/util.h

@@ -147,6 +147,12 @@ static inline int memcmp_safe(const void *s1, const void *s2, size_t n) {
         return memcmp(s1, s2, n);
 }
 
+/* Compare s1 (length n1) with s2 (length n2) in lexicographic order. */
+static inline int memcmp_nn(const void *s1, size_t n1, const void *s2, size_t n2) {
+        return memcmp_safe(s1, s2, MIN(n1, n2))
+            ?: CMP(n1, n2);
+}
+
 int on_ac_power(void);
 
 #define memzero(x,l)                                            \