This allows the user to set the IPv6 Hop Limit value on a packet or
to increment or decrement it by a given value.

Example:
# ip6tables -t mangle -A OUTPUT -j HOPLIMIT --hl-inc 1
# ip6tables -t mangle -A INPUT -j HOPLIMIT --hl-eq 64
# ip6tables -t mangle -A OUTPUT -j HOPLIMIT --hl-dec 2
