debugger: introduce logging facility

It's frequently needed to see interaction of a luatest script
and its chield launched via popen.
Now we display extra debugging information received from
children pipes directly to the parent screen. It's hidden
by default by luatest (if everything goes well), but could
be displayed with `luatest -c` option.

```
luatest -c -v app-luatest/console_debugger_session_test.lua
```

NO_DOC=internal
NO_CHANGELOG=internal

(cherry picked from commit 7a813f43)