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
(cherry picked from commit 14ce0c25)