netfilter: Combine ipt_TTL and ip6t_HL source

Suggested by: James King <t.james.king@gmail.com>

Similarly to commit c9fd4968, merge
TTL and HL. Since HL does not depend on any IPv6-specific function,
no new module dependencies would arise.

With slight adjustments to the Kconfig help text.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>