ndp: Bring back the -A option for the netlink implementation

This was lost during the conversion to netlink.

Reviewed by: kp
Approved by: re (cperciva)
Fixes: 91fbe0819bb9 ndp: convert ndp(8) to netlink
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47234

(cherry picked from commit b6d4e3bee350b881b5efea03068c1247985b12fe)
(cherry picked from commit 0de672a59481d48dd5e4667c597d66e1b042a683)