Expect POSIX compatible vsnprintf.

Fixes #8784

This seems to be a mingw problem. I can reproduce this with the windows-SDL and windows-curses build using wine on linux. It does not happen with the linux build.

My guess: `vstring_format` (output.cpp) uses `vsnprintf` to format the string. That function is supposed to return -1 if the buffer it to small (see http://msdn.microsoft.com/en-us/library/1kt27hek.aspx), in that case the buffer is increased and the function is called again until it succeeds. The function returns a number >= 0 only when the buffer was large enough.

The linux/POSIX version of that function returns the size of the buffer that is required to print the formatted string (even when the current buffer size if to small).

It's a bit unclear to me which version mingw uses. There is code in output.cpp that handles both versions (depending on whether it is compiled for window or not). It also depends on localization: with `-DLOCALIZE` (which seems to be the default), `vsnprintf` is actually `libintl_vsnprintf` (coming from libintl, which is part of gettext, it may or may not be POSIX compatible, that apparently depends on its input, see http://lists.gnu.org/archive/html/bug-gnu-utils/2007-02/msg00058.html).

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The problem boils down to this scenario:

The code assumes `vsnprintf` is the windows version (returns -1 when the buffer is too small).

The initial buffer size is 1024 bytes, large enough for most strings. The text with the recipes is probably larger, therefor the first call to `vsnprintf` fails - the windows version would return -1, the buffer would be increased, the function called again. This does not happen.

Instead `vsnprintf` seems to be the POSIX version and returns the *required* buffer size (for example 1040). The program assumes that formatting was successful (any value >= 0 indicates success) and that the buffer was large enough. It copies the content of the buffer, but it assumes that the returned value is the length of the string in the buffer (in this case 1040). But the buffer is only 1024 bytes large, so it reads data from outside of the buffer.

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I've added a simple check for this scenario: if `vsnprintf` returns a number >= 0, the number is compared to the buffer size. If it is less than (or equal to) the buffer size, it is assumed the formatted string has been successfully written in the buffer (and the buffer was large enough) and the buffers content is returned. This works with both windows and POSIX version of `vsnprintf`.

If the returned number is greater than the buffer size, it is assumed that this is the required buffer size, the buffer is enlarged to that size and `vsnprintf` is called again. This indicates a POSIX version of `vsnprintf`. The windows version would not return a number greater than the buffer size.

If the returned value is negative, it is assumed that the buffer was not large enough, the buffer is enlarged (its size if doubled), and `vsnprintf` is called again. This is the same behavior as before.