CLEANUP: fcgi: Use `istadv()` in `fcgi_strm_send_params`

CLEANUP: fcgi: Use `istadv()` in `fcgi_strm_send_params`

Found manually, while creating the previous commits to turn `struct proxy`
members into ists.

There is an existing Coccinelle rule to replace this pattern by `istadv()` in
`ist.cocci`:

    @@
    struct ist i;
    expression e;
    @@

    - i.ptr += e;
    - i.len -= e;
    + i = istadv(i, e);

But apparently it is not smart enough to match ists that are stored in another
struct. It would be useful to make the existing rule more generic, so that it
might catch similar cases in the future.