windows: simplify a bit (#493)

windows: simplify a bit (#493)

The sys* constants were used in the old golang.org/x/exp/winfsnotify
package on which this is based, but haven't been in use for a long time.

Remove the "one shot", "dir only", and "ignore" options as they were
never set. Also don't send "sysFSQOVERFLOW" as an event if the buffer is
too long; we already send an error and it's just a weird magical number
you get.

This could be simplified greatly more by removing the whole
sysFS… → windows.FILE_NOTIFY… → sysFS… conversions, but that's for
another day.