tests/netlink: mark the vlan(4) test with required module

netlink: add netlink interfaces to if_clone

This change adds netlink create/modify/dump interfaces to the `if_clone.c`.
The previous attempt with storing the logic inside `netlink/route/iface_drivers.c`
did not quite work, as, for example, dumping interface-specific state
(like vlan id or vlan parent) required some peeking into the private interfaces.

The new interfaces are added in a compatible way - callers don't have to do anything
unless they are extended with Netlink.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D39032
MFC after: 1 month

tests: split netlink.py into multiple files to impove maintainability.

This diff does not contain any functional changes.
Its sole purpose is splitting netlink.py into smaller chunks.
The new code simplifies the upcoming generic netlink support
introduction.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39365

netlink: fix vlan interface creation

MFC after: 2 weeks

tests: fix netlink test_dump_ifaces_many test.

Consider only loopback interfaces when counting.
Otherwise, if pf is loaded, 'pflog0' gets added to the vnet,
breaking the test.

MFC after: 2 weeks

netlink: fix interface dump.

The current code missed interface addition when reallocating
temporary buffer.
Tweak the code to perform the reallocation first and add
interface afterwards unconditionally.

Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after: 3 days

netlink: return optional metadata with the operation result.

Some operations like interface creation may need to return metadata
- in this case, interface name - back to the caller if the operation
is successful.
This change implements attaching an `NLMSGERR_ATTR_COOKIE` nla to the
operation reply message via `nlmsg_report_cookie()`.
Additionally, on successful interface creation, interface index and
interface name are returned in the `IFLA_NEW_IFINDEX` and `IFLA_IFNAME
TLVs, encapsulated in the `NLMSGERR_ATTR_COOKIE`.

Reviewed By: pauamma
Differential Revision: https://reviews.freebsd.org/D38283
MFC after: 1 week

testing: fix skipping netlink tests if netlink module is not loaded.

netlink: improve RTM_GETADDR handling.

* Allow filtering by ifa_family & ifa_index.
* Add common RTM_<NEW|DEL|GET>ADDR parser
* Add tests verifying RTM_GETADDR filtering behaviour & output
* Factor out common netlink socket test methods into NetlinkTestTemplate
* Add NLMSG_DONE message handler

Reviewed By: pauamma
Differential Revision: https://reviews.freebsd.org/D37970

netlink: connect netlink tests to the build

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D37708

netlink: improve interface handling

* Separate interface creation from interface modification code
* Support setting some interface attributes (ifdescr, mtu, up/down, promisc)
* Improve interaction with the cloners requiring to parse/write custom
interface attributes
* Add bitmask-based way of checking if the attribute is present in the
message
* Don't use multipart RTM_GETLINK replies when searching for the
specific interface names
* Use ENODEV instead of ENOENT in case of failed RTM_GETLINK search
* Add python netlink test helpers
* Add some netlink interface tests

Differential Revision: https://reviews.freebsd.org/D37668