Allow non-portable features

[PieterD]

Not all operations are available on every platform. Windows doesn't have attrib, kqueue doesn't have create, etc.

I propose a method to determine the capabilities of the library. This would allow us to introduce more events that are not available on every platform, like Close: #22

Perhaps something like err := fsnotify.Capabilities(fsnotify.Create|fsnotify.Write)

Similarly, combined with event filtering in #7, NewWatcher would of course fail if an event is requested that it does not support:
err := fsnotify.NewWatcher("/file/name", fsnotify.Create|fsnotify.Write)

The question then remains, is this filter mandatory? I think it should be.

[nathany]

Interesting.

Another proposal is to have something like Sys() interface{} in FileInfo to retrieve the underlying event data (OS specific bitmask).

That doesn't address filtering though.

[arp242]

Closing as duplicate of #519 (that one is much newer, but has some useful comments).

Also PR at #629.