Filesystem.Monitor: Poll accelerated monitors too.
Imported from https://youtrack.roxen.com/issue/PIKE-15
Reported by @grubba
Related to PIKE-19
Both the Inotify and FSEvents APIs claim that they support
notifications on eg network file systems, and while they will
succeed in notifying on changes performed by the local host
on such file systems, they will not on changes performed by
other hosts. To avoid missing such changes such monitors
need to be scanned too.