revert "connhelper: use ssh multiplexing"

[thaJeztah]

this reverts #2132, which looks to be responsible for flakiness in CI;

closes #2256
closes #2257

https://ci.docker.com/public/blue/organizations/jenkins/cli/detail/PR-2132/3/pipeline

=== Failed
=== FAIL: e2e/image TestBuildFromContextDirectoryWithTag (0.32s)
    build_test.go:37: assertion failed: 
        Command:  docker build -t myimage .
        ExitCode: 1
        Error:    exit status 1
        Stdout:   Sending build context to Docker daemon  4.608kB

        
        Stderr:   error during connect: Post http://docker/v1.40/build?buildargs=%7B%7D&cachefrom=%5B%5D&cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile&labels=%7B%7D&memory=0&memswap=0&networkmode=default&rm=1&shmsize=0&t=myimage&target=&ulimits=null&version=1: command [ssh -o ControlMaster=auto -o ControlPath=/root/.docker/%r@%h:%p -l penguin 172.20.0.2 -- docker system dial-stdio] has exited with exit status 255, please make sure the URL is valid, and Docker 18.09 or later is installed on the remote host: stderr=
        
        
        Failures:
        ExitCode was 1 expected 0
        Expected stderr to contain "[NOTHING]"
        Expected no error

=== FAIL: e2e/image TestPullWithContentTrustUsesCacheWhenNotaryUnavailable (3.39s)
    pull_test.go:51: assertion failed: 
        Command:  docker image rm registry:5000/trust-pull-unreachable:latest
        ExitCode: 1
        Error:    exit status 1
        Stdout:   
        Stderr:   error during connect: Delete http://docker/v1.40/images/registry:5000/trust-pull-unreachable:latest: command [ssh -o ControlMaster=auto -o ControlPath=/root/.docker/%r@%h:%p -l penguin 172.20.0.2 -- docker system dial-stdio] has exited with exit status 255, please make sure the URL is valid, and Docker 18.09 or later is installed on the remote host: stderr=
        
        
        Failures:
        ExitCode was 1 expected 0
        Expected no error

[thaJeztah]

ping @tonistiigi @AkihiroSuda PTAL: perhaps you know what's causing the problem but otherwise I think we should revert for now

[tonistiigi]

@AkihiroSuda Looks like reporting output from the failed ssh helper in that code does not work properly . Not sure if it is in stdout or there is a race on write/read stderr to buffer and program exiting.

[silvin-lubecki]

@tonistiigi do you have any new element on that race?