Display some useful info about your system necessary for bug reports.

Usage:
  v doctor

The report also tells which `vlib` the V executable is paired with:
`V vlib dir` flags a `vlib` that is not in the folder of the V executable, and
`V vlib commit` flags a `vlib` checkout that is not at the commit that V was
built from. Both happen silently, when an executable is copied, or when a
checkout is updated without rebuilding V. In a folder of another V checkout,
the `cwd vlib` rows inspect its `vlib` as well. Which checkout is selected for
compilation depends on the compiler mode.
See doc/vdoctor.md for details.
