showlog: Use localtime() instead of gmtime()

showlog: Use localtime() instead of gmtime()

From the gmtime manpage:
  The gmtime()  function converts the calendar time timep to broken-
  down time representation, expressed in Coordinated Universal Time
  (UTC).

  The localtime() function converts the calendar time timep to
  broken-down time representation, expressed relative to the user’s
  specified timezone.

Since the local timezone is what we want, we should be using
localtime(), which also allows us to get rid of the isdst check
(which never triggered, since gmtime() doesn't set that field).

This fixes issue #3525.

Signed-off-by: Andreas Ericsson <ae@op5.se>