Modify PayPal "no_shipping" var via t-shirt choice

According to
https://developer.paypal.com/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/

no_shipping has the following values:
0: prompt for an address, but do not require one
1: do not prompt for an address
2: prompt for an address, and require one
The default is 0.

Ideally, any time they change wantGift, even in a pure HTML form, we'd
change it between 0 and 2 as appropriate (i.e., we need the address if
they want the t-shirt).

However, I couldn't find an easy way to make this modification in pure
CSS or HTML, so it only happens in Javascript-enabled browsers.

This is still graceful degradation, since the only impact is in cases
where a non-Javascript user fails to give us an address, and we have to
email later to get the shipping address.