Add sysctl descriptions for net.inet6.ip6 and net.inet6.icmp6.

icmp6.redirtimeout, icmp6.nd6_maxnudhint and ip6.rr_prune are left
undocumented as they appear to have no effect. Some existing sysctl
descriptions were modified for consistency and style, and the
ip6.tempvltime and ip6.temppltime handlers were rewritten to be a bit
simpler and to avoid setting the sysctl value before validating it.

MFC after: 3 weeks