output: JSON: fix output of GMT offset

output: JSON: fix output of GMT offset

The compiler has two sets of complaints.  Firstly, `t->tm_gmtoffset` is
a `long int`, but it is being passed to `abs`, which leads to warnings
such as:

  ulogd_output_JSON.c:308:34: warning: absolute value function `abs` given an argument of type `long int` but has parameter of type `int` which may cause truncation of value

Secondly, it can't verify that the hour value derived from the offset
will in fact fit into `%02d`, thus:

  ulogd_output_JSON.c:306:37: warning: `%02d` directive output may be truncated writing between 2 and 6 bytes into a region of size 5

To remedy these, we now mod the offset by 86,400 and assign it to an `int`
before deriving the hour and minute values.

We also change the format-specifier for the hour value to `%+03d` which
causes a sign to be printed even if the value is positive, thus allowing
us not to specify the sign explicitly and to drop the `abs` call for the
hour value.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>