Interface stacking update - 4

Added changes for #interface #stacking handling routines in interfaces module. This will create an individual stacking layer between a pair of #lower and #higher layer interfaces. This will support different kind of checks - such as compatibility checks, capacity verification, etc ... - before and after the stacking layer creation or removal on upper and lower layer interfaces.

#IF module update 5 ...
[commit 09d65607d9ebce84e92c3e5cc9e55c90af8a8224]