coverage: use lcov to generate coverate reports

Type "make coverage" to generate a coverage report and launch it using
firefox.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter: move LogFilterPipe to a separate module

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-op: split off or/and operations from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-cmp: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-tags: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-netmask: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-call: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-re: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter/filter-pri: split off from filter-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
lib/filter: clean up includes

All files should include only the headers required to that specific code.
Since the filter-expr.c file was split, a number of unneeded includes
remained in filter-expr.{c,h}, remove those and make sure the split off files
include what those need.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>