Escape control and nonprintable characters in docker ps

The docker ps command displays the user-entered command running in a container.
If that command contained \n, \t, or other control characters, they were
interpreted literally, and newlines and tabs would be printed in the output.
Escape the command string to make things more readable.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Update documentation noting that SIGCHLD is not proxied.

Docker's --sig-proxy option sends all signals but one to a container. The
exception s SIGCHLD, which is deliberately ignored as it doesn't make sense to
send such a signal to a process in a container. Documentation updates will make
this less confusing if anyone does attempt to do this.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
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)