Fix periodic scripts when an NFS mount covers a local mount

100.chksetuid and 110.neggrpperm try to search through all UFS and ZFS
filesystems. But their logic contains an error. They also search through
remote filesystems that are mounted on top of the root of a local
filesystem. For example, if a user installs a FreeBSD system with the
default ZFS layout, he'll get a zroot/usr/home filesystem. If he then mounts
/usr/home over NFS, these scripts would search through /usr/home.

MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D7482