- trims +100 lines of code from spprintf.c

- trims +100 lines of code from spprintf.c
    - introduces an overflow detection in STR_TO_DEC
    - eliminates dead code (e.g. assert(foo); if (foo) {..})
    - removes unused macros from the original code
    - simplifies code (e.g. cc was completely dropped)
    - improves run-time performance

      The max_len feature is never used in our code base.
      Nevertheless, cpu cycles were spent on each string
      operation to check the current length against max_len which
      is quite inefficient.  Thus, I've moved the check to
      vspprintf where it is applied only once per call.