Review of std.signal

[Dicebot]

Upping the thread as it did not receive much review attention is 
is slowly getting fading away.

Robert, for someone who has only vague knowledge of signal stuff 
(me), can you explain why exactly this design was necessary? 
(mixing in global variable and wrapper function which returns its 
restricted version) Is the desire to separate protection 
attributes the only motivation or there are some other concerns?