Display better message when booting and awaiting package reinstall 

If I login to the webGUI quickly after an upgrade, just as the system is coming up and before it has actually got to the last package reinstall step of the boot script, then the webGUI goes to the page "System: Package Manager: Install Package" with the text "Package: All packages will be reinstalled. Please confirm the action." and "Confirm" and "Cancel" buttons. Going to the dashboard 30-60 seconds later, when the boot script has actually got to the package reinstall step, will display the "normal" dashboard notice about "Packages are currently being reinstalled in the background."
That will be a bit perplexing for users who hit this, and actually we do not want people press "Confirm" at that point, because the boot script is about to do that anyway - we do not want 2 copies of the reinstall script running against each other.
This change checks for that condition and gives a slightly different notice on the dashboard so the user can understand that "pfSense is booting then packages will be reinstalled in the background."
I thought it looked easy to read like that, and I hope parser/pre-compilers optimise-out empty blocks. I will change...