ZEP: use the exissting NTP time formatting code.

ZEP: use the exissting NTP time formatting code.

The strftime() format for that case is slightly different, so add a new
p_ntp_time_fmt() routine that takes a strftime() format, and use that in
ZEP.

That way, we can benefit from any handling of times not fitting in a
time_t being done by the NTP time printing code.