templates: Properly clear options defaults

In log_template_options_defaults(), clear all of the LogTemplateOptions
structure, not just the first few bytes.

Reported-by: Michal Privoznik <miso.privoznik@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
libtest: Use AM_CPPFLAGS instead of AM_CFLAGS

Using AM_CPPFLAGS picks our headers first, so if any headers on the
default include path conflict with syslog-ng headers, the good ones will
be picked.

Reported-by: Michal Privoznik <miso.privoznik@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>