ifconfig: Support VLAN ID in static/deladdr

[ivy]

Summary

Add a new type of ifconfig command, DEF_CMD_VARG, which supports a
variable number of arguments.

Use this to add an optional 'vlan <n>' argument to static and deladdr,
which specify the bridge VLAN ID that the address should be added to
or removed from.

Add tests for the new functionality, and improve the test for the
existing "static" command.

[ivy]

rebase on main and remove from the stack

change the syntax as suggested by @fuz:

% ifconfig bridge0 static <intf> <addr> [vlan <id>]
% ifconfig bridge0 deladdr <addr> [vlan <id>]

this avoids the need to add a new command.

[kevans]

LGTM, but please split this into two: one to add the ARGVECTOR / c_funcv notion, and one to adopt it for static/deladdr. My reasoning here (in general) is that while the latter isn't useful without the former, later changes (however unlikely) could adopt it that might make it useful to MFC in a different timeframe (if even planned) than you're planning for vlan features. It's also a useful addition to highlight independently of your specific use-case here.