cookie_interface: avoid printing time_t directly

[carenas]

time_t representation is undefined and varies on bitsize and signedness,
and as of C11 could be even non integer.

instead of casting to unsigned long (which would truncate in systems with
a 32bit long after 2106) use difftime to get the elapsed time as a double
and print that (without decimals) instead.

alternatively a cast to curl_off_t and its corresponding print formatting
could have been used (at least in POSIX) but portability and curl agnostic
code was prioritized.

[bagder]

Thanks!

[bagder]

Unfortunately, this caused scan-build to yell:

cookie_interface.c: In function ‘main’:
cookie_interface.c:96:53: error: ‘%.0lf’ directive output may be truncated writing between 1 and 310 bytes into a region of size 230 [-Werror=format-truncation=]
     snprintf(nline, sizeof(nline), "%s\t%s\t%s\t%s\t%.0lf\t%s\t%s",
                                                     ^~~~~
cookie_interface.c:96:5: note: ‘snprintf’ output between 70 and 379 bytes into a destination of size 256
     snprintf(nline, sizeof(nline), "%s\t%s\t%s\t%s\t%.0lf\t%s\t%s",
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              ".example.com", "TRUE", "/", "FALSE",
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              difftime(time(NULL) + 31337, (time_t)0),
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              "PREF", "hello example, i like you very much!");
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors