Removed the unsafe nxt_memcmp() wrapper for memcmp(3).

The casts are unnecessary, since memcmp(3)'s arguments are 'void *'.
It might have been necessary in the times of K&R, where 'void *' didn't
exist.  Nowadays, it's unnecessary, and _very_ unsafe, since casts can
hide all classes of bugs by silencing most compiler warnings.

The changes from nxt_memcmp() to memcmp(3) were scripted:

$ find src/ -type f \
  | grep '\.[ch]$' \
  | xargs sed -i 's/nxt_memcmp/memcmp/'

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

src/test/nxt_tests.c

@@ -41,7 +41,7 @@ main(int argc, char **argv)
 #if (NXT_TEST_RTDTSC)
 
     if (nxt_process_argv[1] != NULL
-        && nxt_memcmp(nxt_process_argv[1], "rbm", 3) == 0)
+        && memcmp(nxt_process_argv[1], "rbm", 3) == 0)
     {
         if (nxt_rbtree1_mb_start(thr) != NXT_OK) {
             return 1;