fmt_iso8601 - write a date in ASCII conforming to ISO-8601

       fmt_iso8601  writes  a  date  in  ASCII  representation  as  ISO-8601 defines it: "2014-05-27T19:22:16Z".
       ISO-8601 also allows to store a sub-second part and a time zone, but fmt_iso8601 does not support that.

       fmt_iso8601 does not append \0.

       If dest equals FMT_LEN (i.e. is zero), fmt_iso8601 returns the number of bytes it would have written.

       The return value of fmt_iso8601 is 20 until Jan 1st 10000.

                                                                                                  fmt_iso8601(3)

       fmt_iso8601 - write a date in ASCII conforming to ISO-8601

#include<libowfat/fmt.h>

       size_t fmt_iso8601(char *dest,time_t source);