Fix a variety of bugs in docker-env output

[tstromberg]

Bugfixes:

• Remove errant newline in powershell and cmd output docker-env command does not work in powershell (v1.7) #6524
• Remove unnecessary semicolon in fish output
• Fix corrupt unset output if --unset and --no-proxy are used together
• Remove trailing tab from cmd, powershell, and emacs output
• Remove incorrect comment from unset output

Other changes:

• Now unsets with a single command in compatible environments
• Minor rewording of comment

[tstromberg]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tstromberg

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• OWNERS [tstromberg]

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[minikube-pr-bot]

Error: running mkcmp: exit status 1

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5d59e83). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master    #6540   +/-   ##
=========================================
  Coverage          ?   37.88%           
=========================================
  Files             ?      137           
  Lines             ?     8674           
  Branches          ?        0           
=========================================
  Hits              ?     3286           
  Misses            ?     4975           
  Partials          ?      413

Impacted Files	Coverage Δ
cmd/minikube/cmd/env.go	57.37% <ø> (ø)
pkg/minikube/bootstrapper/bootstrapper.go	0% <ø> (ø)
cmd/minikube/cmd/start.go	22.1% <50%> (ø)

[medyagh]

@tstromberg thank you for fixing up after my human errors