long firestring_estr_sprintfe(struct firestring_estr_t *out, struct firestring_estr_t *format, ...)
DESCRIPTION
firestring_estr_sprintfe() behaves exactly like
firestring_estr_snprintf(3)
except that it takes an estring format argument, so the
format can contain arbitrary binary data. However, please
note that user user-input data as the format string is
INSECURE.