Add support for detach docker exec

  
    This is a follow-on to #9208.  In this one we add:
    - support for (-d) detach mode of docker exec, where we show the execID to
      the cli
    - add support for "docker execwait", which waits for an exec cmd to finish
    - add support for shortIDs when specifying execIDs - meaning a prefix of the
    
    While we could do "execwait" under "wait" I decided to create a new top-leve
    command so that we didn't run into conflicts if (when?) we allow for
    exec commands to be named.
    
    Signed-off-by: Doug Davis <dug@us.ibm.com>
Indent?