[FrameworkBundle] improve server commands feedback

* display a message when `server:start` is executed and the PCNTL
  extension is not loaded
* print instructions about how to terminate the `server:run` command
bug #12253 [FrameworkBundle] improve server commands feedback (xabbuh)

This PR was merged into the 2.6-dev branch.

Discussion
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[FrameworkBundle] improve server commands feedback

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | kind of
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12153
| License       | MIT
| Doc PR        |

* display a message when `server:start` is executed and the PCNTL
  extension is not loaded
* print instructions about how to terminate the `server:run` command

Commits
-------

bf174cf [FrameworkBundle] improve server commands feedback