Add ability to create subint's from Linux

Using the earlier placeholder hint in lcp_nl_link_add(), I know that
I've gotten a NEWLINK request but the linux ifindex doesn't have a LIP.

This could be because the interface is entirely foreign to VPP, for
example somebody created a dummy interface or a VLAN subint on one:
ip link add dum0 type dummy
ip link add link dum0 name dum0.10 type vlan id 10

Or, I'm actually trying to create a VLAN subint, like these:
ip link add link e0 name e0.1234 type vlan id 1234
ip link add link e0.1234 name e0.1235 type vlan id 1000
ip link add link e0 name e0.1236 type vlan id 2345 proto 802.1ad
ip link add link e0.1236 name e0.1237 type vlan id 1000

None of these NEWLINK callbacks, represented by vif (linux interface
id) will have a corresponding LIP. So, I try to create one by calling
lcp_nl_link_add_vlan().

Here, I lookup the parent index ('dum0' or 'e0' in the first examples),
the former of which also doesn't have a LIP, so I bail. If it does,
I still have two choices:
1) the LIP is a phy (ie TenGigabitEthernet3/0/0) and this is a regular
   tagged interface; or
2) the LIP is itself a subint (ie TenGigabitEthernet3/0/0.1234) and
   what I'm asking for is a QinQ or QinAD sub-interface.

So I look up as well the phy LIP. We now have all the ingredients I
need to create the VPP sub-interfaces with the correct inner-dot1q
and outer dot1q or dot1ad.

Of course, I don't really know what subinterface ID to use. It's
appealing to "just" use the vlan, but that's not helpful if the outer
tag and the inner tag are the same. So I write a helper function
vnet_sw_interface_get_available_subid() whose job it is to return an
unused subid for the phy -- starting from 1.

I then create the phy sub-interface and the tap sub-interface,
tying them together into a new LIP. During these interface creations, I
want to make sure that if lcp-auto-subint is on, we disable that. I
don't want VPP racing to create LIPs for the sub-ints right now. Before
I return (either in error state or upon success), I put back the
lcp-auto-subint to what it was before.

If I manage to create the LIP, huzzah. I return it to the caller so it
can continue setting link/mac/mtu etc.

README.md

@@ -48,14 +48,13 @@ column shows changes in LInux that are copied into VPP.
 | Change MTU     | ✅            | ✅            |
 | Change MAC     | ❌ 1)         | ✅            |
 | Add/Del IP4/IP6 Address  | ✅  | ✅            | 
-| MPLS           | ❌            | ❌            |
 | Route          | ❌ 2)         | 🟠            |
 | Add/Del Tunnel | ❌            | ❌            |
 | Add/Del Phy    | ✅            | 🟠            |
-| Add/Del .1q    | ✅            | 🟠            |
-| Add/Del .1ad   | ✅            | 🟠            |
-| Add/Del QinQ   | ✅            | 🟠            |
-| Add/Del QinAD  | ✅            | 🟠            |
+| Add/Del .1q    | ✅            | ✅            |
+| Add/Del .1ad   | ✅            | ✅            |
+| Add/Del QinQ   | ✅            | ✅            |
+| Add/Del QinAD  | ✅            | ✅            |
 | Add/Del BondEthernet  | ✅     | 🟠            |
 
 Legend: ✅=supported; 🟠=maybe; ❌=infeasible.