in those cases:
1. File system was mounted by an unprivileged user.
2. File system was mounted by an unprivileged root user.
3. File system was mounted by a privileged non-root user.
Point 1 is when file system was mounted by unprivileged user
(sysctl vfs.usermount was equal to 1 then).
Point 2 is when file system was mounted by root, while sysctl
security.bsd.suser_enabled is set to 0 and sysctl vfs.usermount
is set to 1.
Point 3 is because we want to be ready for capabilities.
Reviewed by: rwatson
Approved by: scottl (mentor)