fix printing of infinite lifetime in ICMPv6

fix printing of infinite lifetime in ICMPv6

icmp6_opt_print() in the case of ND_OPT_PREFIX_INFORMATION always
appended "s" (seconds) to the returned string result of get_lifetime(),
which could be either a printed number or the "infinite" word. This
change leaves it up to get_lifetime() to decide if "s" is necessary.
This also improves the ND_OPT_ROUTE_INFO case, where route information
lifetime has the same type and semantics.