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/nxt_string.c

@@ -257,7 +257,7 @@ nxt_memstrn(const u_char *s, const u_char *end, const char *ss, size_t length)
                 return NULL;
             }
 
-            if (nxt_memcmp(s, s2, length) == 0) {
+            if (memcmp(s, s2, length) == 0) {
                 return (u_char *) s - 1;
             }
         }
@@ -325,7 +325,7 @@ nxt_rmemstrn(const u_char *s, const u_char *end, const char *ss, size_t length)
         c1 = *s1;
 
         if (c1 == c2) {
-            if (nxt_memcmp(s1 + 1, s2, length) == 0) {
+            if (memcmp(s1 + 1, s2, length) == 0) {
                 return (u_char *) s1;
             }
         }