autounmountd: set filter-specific flags for the EVFILT_FS filter

Set the filter-specific flags VQ_MOUNT and VQ_UNMOUNT for the
EVFILT_FS filter.

The filter-specific flags for the EVFILT_FS filter are undocumented, but
their usage can be found by looking up vfs_event_signal().

Reviewed by: trasz
Differential Revision: https://reviews.freebsd.org/D28975