[settings] Explicitly separate the concept of a completed fetched setting

The fetch_setting() family of functions may currently modify the
definition of the specified setting (e.g. to add missing type
information).  Clean up this interface by requiring callers to provide
an explicit buffer to contain the completed definition of the fetched
setting, if required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[settings] Allow for IPv6 setting types in non-IPv6 builds

Allow for the existence of references to IPv6 setting types without
dragging in the whole IPv6 stack, by placing the definition of
setting_type_ipv6 in core/settings.c and providing weak stub methods
for parse_ipv6_setting() and format_ipv6_setting().

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[settings] Add fetch_ipv6_setting()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[dns] Add support for resolving IPv6 addresses via AAAA records

Our policy is to prefer IPv6 addreses to IPv4 addresses, but to
request IPv6 addresses only if we have an IPv6 address for the name
server itself.

Signed-off-by: Michael Brown <mcb30@ipxe.org>