ip6addrctl(8): Strictly check the number of arguments

The additional arguments are useless but may tempt user a wrong usage.

Reviewed by: dfr
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48701

(cherry picked from commit 13ce55b66fad2b2b4054a89a1a906f58dc33683b)
(cherry picked from commit 99bcb54d8d9138480001a262f080b7e8b0c24f58)