Proposal: user should be able to stop/kill an exec command

Fix #9098

This proposes additional extenstions to the remote API for managing exec commands.
- POST /exec/id/stop similar to POST /containers/id/stop
  this will stop or kill the exec command after timeout
- POST /exec/id/kill similar to POST /containers/id/kill
  this will kill the exec command (optionally with a custom signal)

cc @vishh
also @vieux @jfrazelle for the API change

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)