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_test.c

@@ -59,7 +59,7 @@ nxt_utf8_overlong(nxt_thread_t *thr, u_char *overlong, size_t len)
 
         size = (p != NULL) ? p - utf8 : 0;
 
-        if (len != size || nxt_memcmp(overlong, utf8, size) != 0) {
+        if (len != size || memcmp(overlong, utf8, size) != 0) {
 
             u = 0;
             for (i = 0; i < len; i++) {