MFC r310088, r310090, r310095

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@310380 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f