SO_REUSEPORT

Closes #535.

Tested the router part. Still need to test discoverd.

```
ubuntu@ip-172-31-12-144:~/.go/src/github.com/flynn/flynn$ flynn -a router scale app=2
scaling app: 1=>2

06:03:12.824 ==> app ip1723112144-490a83fe605444d09bd0ff1f6ce352db up

scale completed in 672.778114ms
ubuntu@ip-172-31-12-144:~/.go/src/github.com/flynn/flynn$ flynn -a router log ip1723112144-dd0b7949c24449f3be4aee6ef8f8598d
[router] 2014/12/02 06:02:36.330093 Started GET /
[router] 2014/12/02 06:02:36.330267 Completed 404 Not Found in 192.931µs
[router] 2014/12/02 06:02:36.331816 Started POST /routes
[router] 2014/12/02 06:02:36.333088 Completed 200 OK in 1.282731ms
[router] 2014/12/02 06:02:36.334838 Started POST /routes
[router] 2014/12/02 06:02:36.347672 Completed 200 OK in 12.832373ms
2014/12/02 06:02:36 client read err: read tcp 172.31.12.144:48044: connection reset by peer
[router] 2014/12/02 06:02:36.880774 Started POST /routes
[router] 2014/12/02 06:02:36.898217 Completed 200 OK in 17.441098ms
ubuntu@ip-172-31-12-144:~/.go/src/github.com/flynn/flynn$ flynn -a router log ip1723112144-490a83fe605444d09bd0ff1f6ce352db
ubuntu@ip-172-31-12-144:~/.go/src/github.com/flynn/flynn$ flynn -a router log ip1723112144-dd0b7949c24449f3be4aee6ef8f8598d
[router] 2014/12/02 06:02:36.330093 Started GET /
[router] 2014/12/02 06:02:36.330267 Completed 404 Not Found in 192.931µs
[router] 2014/12/02 06:02:36.331816 Started POST /routes
[router] 2014/12/02 06:02:36.333088 Completed 200 OK in 1.282731ms
[router] 2014/12/02 06:02:36.334838 Started POST /routes
[router] 2014/12/02 06:02:36.347672 Completed 200 OK in 12.832373ms
2014/12/02 06:02:36 client read err: read tcp 172.31.12.144:48044: connection reset by peer
[router] 2014/12/02 06:02:36.880774 Started POST /routes
[router] 2014/12/02 06:02:36.898217 Completed 200 OK in 17.441098ms
ubuntu@ip-172-31-12-144:~/.go/src/github.com/flynn/flynn$ flynn -a router ps
ID                                             TYPE
ip1723112144-490a83fe605444d09bd0ff1f6ce352db  app
ip1723112144-dd0b7949c24449f3be4aee6ef8f8598d  app
```