ip6addrctl(8): Strictly check the number of arguments

[zlei]

Summary

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

MFC after: 1 week

Test Plan

Before this change:

# ip6addrctl show ::/0
Prefix                          Prec Label      Use
::1/128                           50     0        0
::/0                              40     1        0
::ffff:0.0.0.0/96                 35     4        0
2002::/16                         30     2        0
2001::/32                          5     5        0
fc00::/7                           3    13        0
::/96                              1     3        0
fec0::/10                          1    11        0
3ffe::/16                          1    12        0

After the change:

# ip6addrctl show ::/0
usage: ip6addrctl [show]
       ip6addrctl add <prefix> <precedence> <label>
       ip6addrctl delete <prefix>
       ip6addrctl flush
       ip6addrctl install <configfile>

[dfr]

Looks good