The string function in a sprinter may be called with a NULL string
pointer (eg if a header is absent). This causes a segfault. We fix
this by checking for a null pointer in the string functions and update
the sprinter documentation.
At the moment some output when format=text is done directly rather than
via an sprinter: in that case a null pointer is passed to printf or
similar and a "(null)" appears in the output. That behaviour is not
changed in this patch.
This method allows callers to output strings with specific lengths.
It's useful both for strings with embedded NULs (which JSON can
represent, though parser support is apparently spotty), and
non-terminated strings.
Using the new structured printer support in sprinter.h, implement
sprinter_json_create, which returns a new JSON structured output
formatter. The formatter prints output similar to the existing JSON, but
with differences in whitespace (mostly newlines, --output=summary prints
the entire message summary on one line, not split across multiple lines).
Also implement a "structured" formatter for plain text that prints
prefixed strings, to be used with notmuch-search.c plain text output.
