fix Issue 12738 - core.sys.posix.signal sigaction_t handler type mismatch

Fix https://issues.dlang.org/show_bug.cgi?id=12738

The idea is simple - for functions that call a callback, create a `nothrow` overload for function that takes a `nothrow` callback. This has the desired results of:

1. you can call the function in nothrow code, as long as you supply a nothrow callback
2. existing code will continue to compile without change