Remove constant basereachable and retrans members from struct nd_ifinfo

Remove constant basereachable and retrans members from struct nd_ifinfo

Both are initalised with compile-time constants and never written to.

They are part of the Neighbour Discovery machinery and only surface
through the single-user SIOCGIFINFO_IN6:
	$ ndp -i lo0
	basereachable=30s0ms, reachable=39s, retrans=1s0ms

These values are read-only since 2017
sys/netinet6/nd6.c r1.217
usr.sbin/ndp/ndp.c r1.85
    Remove knob and always do neighbor unreachable detection

Inline the macros (to keep meaningful names), shrink the per-interface
allocated struct nd_ifinfo to what is actually needed and inline
nd6_dad_starttimer()'s constant `msec' argument.

Nothing else in base, incl. regress, uses SIOCGIFINFO_IN6 or `ndp -i'.

OK bluhm