json: escape u0000 .. u001F when outputting json
Markus Armbruster pointed out:
JSON requires control characters in strings to be escaped. RFC 4627
section 2.5:
A string begins and ends with quotation marks. All Unicode
characters may be placed within the quotation marks except for the
characters that must be escaped: quotation mark, reverse solidus, and
the control characters (U+0000 through U+001F).
We've been quoting the special escape sequences that JSON defines but we
haven't been encoding the full control character range. This patch fixes that.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>