nd6: use CARP link level address in SLLAO for NS sent out

When sending an NS, check if we are using a IPv6 CARP address
and if we do, then put proper CARP link level address into
ND_OPT_SOURCE_LINKADDR option and also put PACKET_TAG_CARP tag
on the packet. The latter will enforce CARP link level address
at the data link layer too, which might be necessary for broken
implementations.
The code really follows what NA sending code has been doing since
introduction of carp(4). While here, bring to style(9) the whole
block of code.

PR: 193280
Differential revision: https://reviews.freebsd.org/D33858