ReadSingleEvent: fixes and improvements (#22)

* ReadSingleEvent: fix buffer size

According to inotify(7),

> Specifying a buffer of size
>
>          sizeof(struct inotify_event) + NAME_MAX + 1
>
> will be sufficient to read at least one event.

Fix the size accordingly.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

* ReadSingleEvent: use unix.Read

This calls the read(2), so it is as good as C.read()
but without a need for cgo.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

* ReadSingleEvent: fix error handling

Since read(2) returns -1 in case of error, the actual reading error
was never returned because the bytesRead < unix.SizeofInotifyEvent
check was performed first.

Reverse the order of checks to fix.

Drop "else" since it's not needed after 'return'.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Remove unnecessary return err in NewWatcher

Merge branch 'testing'