Add client side support for DNS configuration. Use RTM_PROPOSAL_STATIC

Add client side support for DNS configuration. Use RTM_PROPOSAL_STATIC
route messages to propose the name server to resolvd(8).
For now, iked will only propose a single name server from the first
established connection.

Automatic name server configuration is enabled by default for policies using
the 'iface' option.

discussed with deraadt@
ok for the DNS parts florian@
ok for the rest patrick@