net: core: Support UID-based routing.

This contains the following commits:

1. 0149763 net: core: Add a UID range to fib rules.
2. 1650474 net: core: Use the socket UID in routing lookups.
3. 0b16771 net: ipv4: Add the UID to the route cache.
4. ee058f1 net: core: Add a RTA_UID attribute to routes.
    This is so that userspace can do per-UID route lookups.

Change-Id: Ibb96fe3a7ade5f206cbb5867274f5362d7233f4a