core: Add API to retrieve mtu size

Adding an API to libnetutils to retrieve mtu size of an interface
Made few utility functions from ifc_utils public

Change-Id: Iddec2dcd9f241b146d7f362e2df0724cf740d1f2
(cherry picked from commit 108cded7d96cc347f36b5b78c404c5ebccce13d7)

Conflicts:
	libnetutils/ifc_utils.c