Several `nothrow` things

1. Not all extern blocks are marked with nothrow. Namely, only curses.d has one, the rest do not.
2. Not all function arguments and fields are nothrow. In forms API, all of the structs' fields can hold a throwing function. In curses.d, vidputs and ripoffline can accept throwing functions as well, since nothrow on the enclosing block is not applied transitively to the argument functions.

The first point is very easy to fix, and I will make a pull request shortly. The second one is not, fixing it will break existing code. Not fixing it is possibly dangerous, I'm not sure what the C side would do if a function passed to it throws.