proto: add point-to-point IPv4 address config support (FS#1037)

proto: add point-to-point IPv4 address config support (FS#1037)

Add config support support for point-to-point IPv4 addresses by providing
the uci parameter ptpaddr. This allows to support a gateway being in a
different subnet than the assigned IP by modeling the local IP having a
point-to-point address.
This is similar to the point-to-point IPv4 address support already present
for the protocol handlers.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>