[feature] Output summary of toolset used and compiler command

(Initially discussed on https://cpplang.slack.com/archives/C27KZLB0X/p1568727581248500)

I think dumping toolset and compiler command in the configuration summary at the top of the `b2` output would be already a big usability improvement, clearly indicating what has been determined, what is being used, e.g.

```
$ b2 toolset=clang-7
Summary

- toolset : clang-7
- compiler : /usr/bin/clang++

Performing configuration checks

- default address-model : 64-bit (cached)
- default architecture : x86 (cached)
- symlinks supported : yes (cached)
...patience...
```

Having the actual compiler command displayed is very useful.
It would also allow a less experienced user of `b2` to catch issues (confusions) like this

```
b2 toolset=clang-6
b2 toolset=clang-7
b2 toolset=clang-8
```

where all builds use the very same `clang++`.