Fix failure during enable/disable of features

Fix failure during enable/disable of features

vnet_feature_enable_disable_with_index() checks the
return status of vnet_config_{add,del}_feature().
If the config string heap index returned is the same
index that was in use prior to the add/delete, it is
concluded that a failure occurred and processing of
the feature stops.

Sometimes the config index that is returned
can legitimately be the same index that was in used
before the add/delete. The old list of features can
have its heap entry deallocated before a new entry for
the new list is allocated. The heap entry for the new
list can be the entry that was deallocated while
deleting the old one.

Make vnet_config_{add,del}_feature() return ~0 on
failure. Look for that return value as an indication
that an error occurred in
vnet_enable_disable_feature_by_index().

Change-Id: I88bb3ff88a76971c1b5e5ece74784ce8ba78373c
Signed-off-by: Matthew Smith <mgsmith@netgate.com>