Consider using information from /proc

[hannobraun]

From the inotify man page:

The set of watch descriptors that is being monitored via an inotify file descriptor can be viewed via the entry for the inotify file descriptor in the process's /proc/[pid]/fdinfo directory. See proc(5) for further details.

We're not currently using this information, but it might be useful. For example, it might be possible to detect whether inotify_add_watch added a new watch or updated an existing one. This might have implications for #74.

I'm not currently aware what kind of performance overhead is associated with access to the /proc filesystem. This should be a consideration when making a decision about this issue.