As preparation for OCI support in nspawn, let's add a JSON parser.
The json.h file contains an explanation why this is new code instead of
just us linking against an existing JSON library.
This was used by the dkr logic, which is gone now, hence remove this too.
Should we need it one day again the git history never forgets...
Note that this only covers the JSON parser. The JSON generator used by
"journalctl -o json" remains, as its much much simpler and requires no
infrastructure except printf() and the most basic escaping.
