libipsec: Also install routes without preferred source to forward traffic

libipsec: Also install routes without preferred source to forward traffic

Fixes a problem installing policies with source traffic selectors that do
not contain/match any of the local addresses.

When installing a route for a source TS that does contain one or more
local addresses, a preferred source address should be set in the route
(analogous to the `src` option in `ip route add`). This address is used
when the host itself sends traffic via that route (in contrast to
forwarding it).

When the source TS does not encompass any of the local addresses, the
host is not allowed to send traffic via this route and does not need to
set the preferred source address. However, the code would just return a
failure because it could not find a matching local address. This commit
changes this so routes without preferred source IP are installed to
allow non-local traffic to get forwarded via TUN device.

Closes strongswan/strongswan#1766