filter-call: propagate modify value in filter() subexpressions

Whether a filter changes the message is a required information to
decide whether to clone a message when entering a log path. When
using the filter() expression within a filter, this information wasn't
properly propagated, which is fixed by this patch.

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>
filter-re: fix match() filter initialization

The match() subexpression is a tweaked version of the generic FilterRE
class, however the constructor didn't properly initialize the embedded
FilterRE superclass. And the open coded version of the initialization
lacked setting up the init virtual method, causing the modified flag
not to be properly initialized in case store-matches was specified.

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>