Implement a loopback instance allocation scheme.
To support creating loopback interfaces with a specific
instance number, a new CREATE_LOOPBACK_INSTANCE API call
with flag is_specified and value user_instance is introduced.
Presumably the existing CREATE_LOOPBACK API message will be
obsoleted and revmoved.
The VAT cli commands can now mention and format
the new field as 'instance %d' data. If no instance
number is named, the old call CREATE_LOOPBACK is used
to maintain backward compatibility. However, if the
instance is named, the new CREATE_LOOPBACK_INSTANCE
message will be used.
Both the dynamically allocated and user-requested instance
number are tracked in a bitvector. If is_specified is 0,
the next free instance will be used.. A request for a specific
instance number will be granted if it is available. On error,
the value ~0 is returned.
Change-Id: I849815563a5da736dcd6bccd262ef49b963f6643
Signed-off-by: Jon Loeliger <jdl@netgate.com>