vfscore: Support closing STD{IN,OUT,ERR} file descriptors

[clupuishere]

The initial release of vfscore library did not introduce closing of standard file descriptors. This is also mentioned in the code here. The underlying structure for standard file descriptors is statically allocated, unlike ordinary file descriptors. A proper fix should allocate these dynamically as well.

[skuenzer]

I guess we would need to introduce a destructor table where libraries can register clean-up work like this. The dtors would get called on machine shutdown.

[clupuishere]

@skuenzer This has nothing to do with cleaning up on shutdown. It just fixes calls such as close(STDIN_FILENO) when you just want to reuse the file descriptor.

[skuenzer]

In such a case, I expect that we get a bit more clean solution with introducing fdtable: #52 . STDIN, STDOUT, STDERR should register a close fd-handler.