[SV 40361] Don't use vsnprintf(), which is an ISO C99 function.

[SV 40361] Don't use vsnprintf(), which is an ISO C99 function.

* output.c (error, fatal, message): Take an extra argument specifying
how many bytes are used by the formatted arguments.
(get_buffer): New function that allocates the requested buffer size.
Remove msc_vsnprintf(), vfmtconcat(), and fmtconcat() as unneeded.
* makeint.h: Declare various helper macros for generating output.
* *.c: Change all error(), fatal(), message() calls to use the macros,
or pass the extra length argument directly.