src: support for timeout never in elements

src: support for timeout never in elements

Allow to specify elements that never expire in sets with global
timeout.

    set x {
        typeof ip saddr
        timeout 1m
        elements = { 1.1.1.1 timeout never,
                     2.2.2.2,
                     3.3.3.3 timeout 2m }
    }

in this example above:

 - 1.1.1.1 is a permanent element
 - 2.2.2.2 expires after 1 minute (uses default set timeout)
 - 3.3.3.3 expires after 2 minutes (uses specified timeout override)

Use internal NFT_NEVER_TIMEOUT marker as UINT64_MAX to differenciate
between use default set timeout and timeout never if "timeout N" is used
in set declaration. Maximum supported timeout in milliseconds which is
conveyed within a netlink attribute is 0x10c6f7a0b5ec.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>