ifconfig: Add format shortcuts.

MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D45166

(cherry picked from commit 847ef59d4b5eab234bd1f8eb947ad74bdab5614e)

ifconfig: Markup nits.

MFC after: 3 days
Reviewed by: imp, allanjude
Differential Revision: https://reviews.freebsd.org/D45209

(cherry picked from commit 42b28f815214aa582fe4ca707687d3af47850230)
(cherry picked from commit dd1a16c9e2c1c95d9ac7ad0b082248e60ede0e99)

Approved by: re (cperciva)