timedatectl: stop using xstrftime

When using strftime in arbitrary locales, we cannot really say how big the
buffer should be. Let's make the buffer "large", which will work fine pretty
much always, and just print n/a if the timestamp does not fit. strftime returns
0 if the buffer is too small and a NUL-terminated string otherwise, so we
can drop the size specifications in string formatting.

$ export LANG=fa_IR.UTF-8
$ date
چهارشنبه ۱۸ اكتبر ۱۷، ساعت ۱۰:۵۴:۲۴ (+0330)
$ timedatectl
Assertion 'xstrftime: a[] must be big enough' failed at ../src/timedate/timedatectl.c:105, function print_status_info(). Aborting.

now:

$ timedatectl
        Local time: چهارشنبه 2017-10-18 16:29:40 CEST
    Universal time: چهارشنبه 2017-10-18 14:29:40 UTC
          RTC time: چهارشنبه 2017-10-18 14:29:40
…

https://bugzilla.redhat.com/show_bug.cgi?id=1503452

src/basic/time-util.h

@@ -148,10 +148,6 @@ clockid_t clock_boottime_or_monotonic(void);
 
 usec_t usec_shift_clock(usec_t, clockid_t from, clockid_t to);
 
-#define xstrftime(buf, fmt, tm) \
-        assert_message_se(strftime(buf, ELEMENTSOF(buf), fmt, tm) > 0, \
-                          "xstrftime: " #buf "[] must be big enough")
-
 int get_timezone(char **timezone);
 
 time_t mktime_or_timegm(struct tm *tm, bool utc);