Fixed the Linux build

System.FSNotify.Linux now compiles with 'removeWatch'. And I've tested it lightly. It passed those tests.

It would probably be a good idea to document the limitation of inotify, that each unique (WatchManager,FilePath) pair will only have one WD. Or maybe it'd be better to document this as a generic limit of FSNotify, in which case you could always use FilePath instead of WatchID.