Making -Xc illegal for modern mode not good in movie

Running the spinning Earth **movie** examples now gives this error:

```
gmt movie 2_earth.sh -C360p -T91 -M55,pdf -N2_earth
coast [ERROR]: Centered (-Xc) and fixed (-Xf) shifts are not available in modern mode
```
While this makes sense for modern mode in general (infinite paper size...) it makes no sense for **movie** since here we actually have a specific paper size. So somehow we need to allow that when a **movie** script is running. Of top of my head here are some options:

1. Let **movie** set a environmental parameter and we check for that before disallowing **-Xc -Xf**
2. Let **movie** (as it converts the _main.sh_ to the actual script that runs in parallel) detect **-Xc|f** and append an undocumented **!** which means "accept this option even in modern mode".
3. Introduce **-XC** and **-XF** for this purpose in **movie** scripts.

I think (2) is pretty simple to implement and does not require users to do anything different (and is backward compatible). Option (3) fails that.

Thoughts?