MFC r310088, r310090, r310095

r310088:
Put the undocumented df feature of mounting filesystems from device
nodes
under an ifdef. Leave enabled.

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513

r310090:
Mount filesystems without executable permissions since they should never
be used.

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513

r310095:
Use nmount(2) rather than the obsolete mount(2).

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513