Every time printf flushes the buffer to us in sprintf() or snprintf(), we call strncat to append the data.
But we want to start from the beginning in the first flush. What if there was data already there?
Well, we just append to the old data. Which is not good, and breaks snprintf()'s maximum size policy.
This fix sets the first byte of str to NULL, to avoid this.
%m as a format specifier is a nonstandard glibc extension, but I like it so I'm implementing it.
What it does is print the value of strerror(errno), without consuming any arguments to printf().
