ndp: improve -c and -d handling in ndp_netlink

[rcm]

Summary

This patch restores/fixes some of the behavior present in pre-netlink ndp(8).

1. Deleting a local address now correctly returns EPERM (instead of ENOENT)
2. ndp -c no longer dumps the entire table while complaining about local addresses
3. Return exit code when deleting entry (e.g. trying ndp -d on a local address is an error)

There is still an issue with ndp -s creating duplicate entries. That will be addressed in a separate patch.

Sponsored by: Rubicon Communications, LLC ("Netgate")