Removed the unsafe nxt_memchr() wrapper for memchr(3).

The casts are unnecessary, since memchr(3)'s argument is 'const 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_memchr() to memchr(3) were scripted:

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

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

src/nxt_h1proto.c

@@ -2609,7 +2609,7 @@ nxt_h1p_peer_header_parse(nxt_http_peer_t *peer, nxt_buf_mem_t *bm)
         p += 12;
         length -= 12;
 
-        p = nxt_memchr(p, '\n', length);
+        p = memchr(p, '\n', length);
 
         if (nxt_slow_path(p == NULL)) {
             return NXT_AGAIN;