Allow "SIG" prefix on signal names in `docker kill` ("SIGKILL", etc)

This way, we can use both `docker kill -s INT some_container` and `docker kill -s SIGINT some_container` and both will do nice things for us. :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)