[netdevice] Separate concept of scope ID from network device name index

[netdevice] Separate concept of scope ID from network device name index

The network device index currently serves two purposes: acting as a
sequential index for network device names ("net0", "net1", etc), and
acting as an opaque unique integer identifier used in socket address
scope IDs.

There is no particular need for these usages to be linked, and it can
lead to situations in which devices are named unexpectedly.  For
example: if a system has two network devices "net0" and "net1", a VLAN
is created as "net1-42", and then a USB NIC is connected, then the USB
NIC will be named "net3" rather than the expected "net2" since the
VLAN device "net1-42" will have consumed an index.

Separate the usages: rename the "index" field to "scope_id" (matching
its one and only use case), and assign the name without reference to
the scope ID by finding the first unused name.  For consistency,
assign the scope ID by similarly finding the first unused scope ID.

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