datatype: validate port number in inet_service_type_parse

datatype: validate port number in inet_service_type_parse

At present, nft accepts out of range port values such as in this example:

    nft add rule ip filter input tcp dport 123456 accept

Attached patch adds checks for both integer overflow and 16 bit overflow,
and avoids getaddrinfo call in the (common) case of digit input. Example
above now produces this output:

    <cmdline>:1:36-41: Error: Service out of range
    add rule ip filter input tcp dport 123456 accept
                                       ^^^^^^
Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>