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_var.c

@@ -456,7 +456,7 @@ nxt_var_next_part(u_char *start, size_t length, nxt_str_t *part,
 
     end = start + length;
 
-    p = nxt_memchr(start, '$', length);
+    p = memchr(start, '$', length);
 
     if (p == start) {
         *is_var = 1;