MFC r310501:

Be more strict about IpAddress type in snmp_value_parse(..)

• Use inet_pton with AF_INET instead of doing longhand with sscanf.
• Use gethostbyname2 with AF_INET to ensure that the hostname isn't accidentally parsed with another address family, e.g. AF_INET6.

NB: IpAddress per RFC-2578 is IPv4 only. Work is in progress to add

the InetAddress type and friends documented in RFC-4001 and
elsewhere (which supports IPv4, IPv6, and more).