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.h

@@ -41,7 +41,7 @@ typedef enum {
 nxt_inline nxt_bool_t
 nxt_is_var(nxt_str_t *str)
 {
-    return (nxt_memchr(str->start, '$', str->length) != NULL);
+    return (memchr(str->start, '$', str->length) != NULL);
 }