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_utf8_file_name_test.c

@@ -129,7 +129,7 @@ nxt_utf8_file_name_test(nxt_thread_t *thr)
 
             nxt_file_close(&task, &lc_file);
 
-            if (n != 4 || nxt_memcmp(utf8, test, 4) != 0) {
+            if (n != 4 || memcmp(utf8, test, 4) != 0) {
                 nxt_log_alert(thr->log, "nxt_file_read() mismatch");
 
                 nxt_file_delete(lc_file.name);