Increase size of buffer for signals

The chan struct used to forward signals to containers was one element only,
which caused some signals to be dropped when many were being received.
Increasing the size of the chan buffer makes this much less likely to happen.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Merge pull request #6508 from mheon/signals

Increase size of buffer for signal forwarding with --sig-proxy