Make regress mpath more reliable.

Create 100 IP addresses and 100 multipath routes. Then the test
can expect a better distribution of routes that are actually used.

OK anton@

Add regression test for multipath routing.

Test that routes set with the -mpath flag distribute the traffic.
Each route has a different gateway and the UDP packets have different
source addresses.

Add regression test for multipath routing.

Test that routes set with the -mpath flag distribute the traffic.
Each route has a different gateway and the UDP packets have different
source addresses. The destination address is identical for all
routes and packets. Analyze the use counter in netstat -r output
to check that multiple routes have been used. Currently this works
only for IPv4. sysctl net.inet.ip.multipath and net.inet6.ip6.multipath
are enabled before testing and reset afterwards. All routes and
interface addresses are created on loopback in a separate routing
domain.

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