interfaces: add socketcall() to the network/network-bind interfaces

[mvo5]

Upon investigating the autopkgtest failure of snapd on i386 (https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/i386/s/snapd/20160519_224511@/log.gz) I found that we get a seccomp denial for syscall 102 which is:

$ scmp_sys_resolver 102
socketcall

Running the test with this branch via CGO_ENABLED=1 GOARCH=386 go run integration-tests/main.go --ip localhost -port 11022 --filter networkBindInterfaceSuite --snappy-from-branch make the integration tests work on i386.

Some more changes were needed because the failure mode of the network integration tests now include getting killed from seccomp. So instead of using c.ExecCommand() I switched to c.ExecCommandErr() where failures are expected.

[jdstrand]

@tyhicks - we said before that since --devmode is available as well as a workaround for people to adjust the filter directly for testing confinement, we would not allow socketcall and instead wait on the seccomp changes which is high priority but behind other high priority work. However, now socketcall is getting in the way of integration tests. Unless we add new code to add socketcall conditionally, I think we have to allow this now. The nice thing though is that with these integration tests we have an easy way to make sure we don't regress when removing socketcall again with the new seccomp.

[jdstrand]

Please feel free to commit the change and we'll undo it in a future commit.