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 ID

While we could do "execwait" under "wait" I decided to create a new top-level
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>