ppp : Unnumbered support

ppp : Unnumbered support

Adds PPP unnumbered support via the parameter unnumbered which points to a logical OpenWRT interface.
The PPP proto shell handler will "borrow" an IP address from the unnumbered interface (if multiple
IP addresses are present the longest prefix different from 32 will be "borrowed") for which a host
interface dependency will be created. Due to the host interface dependency the PPP unnumbered interface
will only "borrow" an IP address from an interface which is up.
The borrowed IP address will be shared as local IP address by the PPP daemon and no other local IP
will be accepted from the peer in the IPCP negotiation.

A typical use case is the usage of a public IP subnet on the Lan interface which will be shared
by the PPP interface as local IP address.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45948 3c298f89-4303-0410-b956-a3cf2f4a3e73