We escape only characters below 32, plus " and \. UTF-8 should be apssed
through verbatim. Since char can be signed (and usually is), the check
broke and happened to escape UTF-8 encoded bytes too. This broke UTF-8
completely.
Note that we don't check for broken or invalid UTF-8, such as described
both in the client API and IPC docs.
Fixes#1874.
(cherry picked from commit f77e3cbf0c)