Fix init.d script to correctly set web server PID

What does this MR do?

Closes #213531 (closed)

With the introduction of Puma, we forgot to update the init.d script used to run GitLab from source to query the correct pidfile during startup (it was still writing a unicorn.pid.)

This MR fixes this by choosing the pidfile name dynamically based on which web server was requested.

I also simplified the config somewhat by dropping the USE_UNICORN switch; we already have and use use_web_server which I use directly now instead. I also make sure to fall back to a sensible default if these values are not set for some reason.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

This is difficult to test since it would require to set up a from-source installation which is a lot of work. I ran partial manual tests by exit-ing the init script early and just making sure the affected logic now triggerd correctly based on the environment variables that are set.