ip/tunnel: Correct and unify ttl/hoplimit printing

ip/tunnel: Correct and unify ttl/hoplimit printing

Both ttl/hoplimit is from 1 to 255. Zero has special meaning:
use encapsulated packet value. In ip-link(8) -d output this
looks like "ttl/hoplimit inherit". In JSON we have "int" type
for ttl and therefore values from 0 (inherit) to 255.

To do the best in handling ttl/hoplimit we need to accept
both cases: missing attribute in netlink dump and zero value
for "inherit"ed case. Last one is broken since JSON output
introduction for gre/iptnl versions and was never true for
gre6/ip6tnl.

For all tunnels, except ip6tnl change JSON type from "int" to
"uint" to reflect true nature of the ttl.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>