async: getSystemHandle() to retrieve associated IOCP handle from dispatcher.

[cheatfate]

Add procedure getSystemHandle() for Windows part of asyncdispatch.nim.
Such procedure allow to access IOCP port handle and create more flexible async solutions, without dirty hacks.

[dom96]

Could we also offer the same thing for epoll/kqueue?

[cheatfate]

@dom96, there is a problem with selectors.
If we return selectors handle, then selectors must implement all possible variants of usage, if we return epoll/kqueue descriptor, then selectors can have problems with unsupported flags, like EVFILT_TIMER and others.

Unix is too stupid, for real it has only 2 events, read and write. So for Linux almost all cases can be easily implemented with addRead/addWrite. BSD/MacOS is harder because we need to handle many flags, EVFILT_TIMER, EVFILT_PROC, EVFILT_SIGNAL, so we need to improve selectors module to hande all of this flags.

Windows is more flexible, but from my experience i'm always needed this IOCP handle.

[dom96]

Even if it's not as useful for epoll/kqueue let's offer it. I like to keep the interface exposed by asyndispatch consistent across all operating systems.

[cheatfate]

Your request will cause changes in selectors.nim, because descriptors of kqueue and epoll is hidden too.
But its not a big problem, but what i must do with plain select part of selectors.nim? And how people know what type of descriptor they are retreiving? epoll? or plain select? or kqueue?

[cheatfate]

@dom96, i dont understand your problem.

When i ask you about this PR in IRC you answered "Sounds good", but when i'm pushing it, then you are not ready to accept it.

You know, i can easily close this PR, because i can use hack to achieve same result, but others no.

[dom96]

Sorry, but just because I say "sounds good" in IRC does not guarantee that I will merge your PR.

This is a standard library and it requires some standards, if we add things on a whim without considering the implications we will end up with a standard library that is difficult to work with.

[cheatfate]

I'm closing PR because of @dom96 position, which i cannot accept.