FIX: random allocated b2g debugger remoteport saved as connect page submit side-effect

Trying to use the new experimental Browser Debugger in Firefox Nightly,
I notice that it doesn't work on my profile because connect.xhtml saved
random allocated port as a side effect of the submit action:

https://mxr.mozilla.org/mozilla-central/source/browser/devtools/framework/connect/connect.js#55

proposed changes to workaround this bug:
- restore previously saved remote debugger port on connect.xhtml tab close
- on install/upgrade restore standard remote debugger port (6000) if previously
  installed version < 3.0pre5