netinet6: use memcpy/memset/memcmp for consistency

[vinicius_ferrao.net.br]

Summary

Convert the remaining bzero()/bcopy()/bcmp() calls in the neighbor
discovery handlers to memset()/memcpy()/memcmp(), and drop the now
redundant caddr_t casts.

This cleanup is to keep code consistent with:
https://cgit.freebsd.org/src/commit/?id=bac4760524a2a15ce75e35251f0c9cdf31732f3f

Code path that isn't affected by the commit is left untouched.

No functional change intended.

Sponsored by: VersatusHPC

Test Plan

Rebuild of GENERIC with -Werror

[glebius]

Thanks! Note that when doing such cleanups it is important to pay attention to the fact that bcopy(9) allows overlapping memory regions and memcpy(9) does not. In this particular patch everything looks fine.