Add ability to backup/repair when clobbering F# SDK, improve error reporting of update-vsintegration.cmd

[abelbraaksma]

The update-vsintegration.cmd is currently (probably) little used, because it is so hard to do a rollback. The instruction in DEVGUIDE.md say "do a repair is the currently the only way to undo those changes", but as reported in #3669, not even a Repair will save your day.

So, I sat to work to add the following things in this PR:

Mini RFC:

• Add option to backup, aptly named backup
• Add option to restore, aptly named restore. Restore should:
  • Copy back the files that were replaced
  • Remove files that were added by the script, but were not in original backup
  • Apply strong-naming again to the official MS-build SDK
  • Do this for SDK, general assembly references, portable profiles
• Make output of running this script readable
• Test for presence of Administrator privileges (elevated command prompt)
• Check if files are in use, or other access-denied errors
• Add a concise report with warnings / errors
• Add some helpful coloring to the logmessages

Hopefully, when that's done, it will be more helpful and, in particular, make it easier for fellow-developers to test each other's changes without too many painful clobbering and unclobbering by hand, or, as in my case, staying in the unknown of whether or not your current installation is stable, or the one you thing it is.

Also related to the effort driven through #3645. With help by @KevinRansom and others and their patience through #3664.

This may even bring it back on the table by adding it to the single build options for build.cmd, see #904.

[abelbraaksma]

fixed as proposed