Add option to bypass NSM TLS checks on local networks

Add option to bypass NSM TLS checks on local networks

* lisp/net/net-utils.el (nslookup-host-ipv4, nslookup-host-ipv6,
  ipv6-expand): New functions to lookup IPv4 and IPv6 addresses from
  DNS.

* lisp/net/nsm.el (nsm-trust-local-network, nsm-should-check,
  nsm-check-tls-connection, nsm-check-plain-connection): New defcustom
  `nsm-trust-local-network' lets users customize whether NSM should
  check for TLS problems when connecting to the hosts on their local
  networks.  `nsm-should-check' determines whether
  `nsm-check-tls-connection' and `nsm-check-plain-connection' should
  perform checks.  localhost is implicitly trusted, thus checks are
  never performed there.