Add make_valid_future

[viboes]

Some functions require that the future are valid.

The needs to check for the validity before using these futures.

if (fut.valid()) {
  auto x = fut.then(fct);
}
if (f1.valid() && f2.valid()) {
  auto x = when_all(f1, f2);
}

An alternative is to provide a make_valid_future function that ensure that the result is valid. If the input future is valid it return the same future, otherwise it sets an exception future_error, and the corresponding error condition would be std::future_errc::no_state.

In this way the user can always use it as

auto x = make_valid_future(fut).then(fct);

auto y = when_all(make_valid_future(f1), make_valid_future(f2));

[viboes]

​https://github.com/boostorg/thread/commit/bb72eadbd8fb11a2ae4d7d1999b72015e9e48012