api: Add a Signal command
api: Add a Signal command

This command can be issued by shims to send signals to the process
inside the VM. It also includes SIGWINCH messages where the new terminal
geometry is sent along in the payload.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

clearcontainers/proxy,dlespiau/clearcontainers-proxy,dlespiau/proxy,dlespiau/proxy,dlespiau/clearcontainers-proxy,clearcontainers/proxy