nullfs: Improve f_flags in statfs().

Include some flags of the nullfs mount itself:
MNT_RDONLY, MNT_NOEXEC, MNT_NOSUID, MNT_UNION, MNT_NOSYMFOLLOW.

This allows userland code calling statfs() or fstatfs() to see these flags.
In particular, this allows opendir() to detect that a -t nullfs -o union
mount needs deduplication (otherwise at least . and .. are returned twice)
and allows rtld to detect a -t nullfs -o noexec mount as noexec.

Turn off the MNT_ROOTFS flag from the underlying filesystem because the
nullfs mount is definitely not the root filesystem.

Reviewed by:	kib
MFC after:	1 week
Revert the part of r247600 which turned the overtemperature and power fail
interrupt shutdown handlers into filters. Shutdown_nice(9) acquires a sleep
lock, which filters shouldn't do. It also seems that kern_reboot(9) still
may require Giant to be hold.

Submitted by:	bde
- Revert the part of r247601 which turned the overtemperature and power fail
  interrupt shutdown handlers into filters. Shutdown_nice(9) acquires a sleep
  lock, which filters shouldn't do. It also seems that kern_reboot(9) still
  may require Giant to be hold.
- Correct an incorrect argument to shutdown_nice(9).

Submitted by:	bde