Description of problem:
When I send signals 1, 17, 27 to the `docker attach` process, which is running with --sig-proxy=true, it's not forwarded. Other signals are...

Version-Release number of selected component (if applicable):
docker-io-0.9.1-1.fc21.x86_64

How reproducible:
always

Steps to Reproduce:
1. docker -D run --detach=true --tty=false --sig-proxy=true --name test_BnHj localhost:5000/ldoktor/fedora:latest bash -c 'echo "STARTED"; for NUM in `seq 1 64`; do trap "echo Received $NUM, ignoring..." $NUM; done; while :; do sleep 1; done'
2. docker -D attach --sig-proxy=true test_BnHj
3. ps ax |grep docker
4. kill -1 $PID

Actual results:
(nothing)

Expected results:
Received 1, ignoring...

Additional info:
When you send any other signal (apart from 19, 9 and the mentioned ones) it works fine.