Add printf attributes in exported headers

Add printf attributes in exported headers

gcc (and other compilers) sometimes generate spurious warnings, and
thus users of public headers must be able to disable warnings.

Printf format attributes can be disabled by setting
    #define _sd_printf_attr_
before including the header file.

Also, add similar logic for sentinel attribute:
    #define _sd_sentinel_attr_
before including the header file disables the attribute.