Suggest a safe workaround for installing Vagrant.

While we're at it, provide code that can be copy-pasted for the "add
sources/pinning" instructions, to make the setup even easier. We also
fix the sections so it's clearer what should be done for each
supported Debian release.
Quote code more consistently.

Nowhere else are we using the $/# convention.
Remove workaround instructions for #6514.

That bug was fixed almost a year ago, and the current instructions
makes it look like you always should build with all proxy caching
disabled.
Update Vagrant memory usage figures.