db-parser: move inherit_properties to the content field where it belongs

This field was erroneously in the PDBAction struct whereas it should have
been part of its "content" member, as it is completely RAC_MESSAGE specific.
There are currently no other action types though.

The patch also renames inherit_properties to inherit_mode in preparation of
the "context" inherit mode.

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>
db-parser: refactor pdb_action_generate_default_message() to smaller functions

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>
db-parser: extract pdb_context_get_last_message()

Instead of peeking into PDBContext's message list to get the last message,
create a designated function for this purpose.

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>
db-parser: use explicit enum references in the data structure

Following a review comment by Viktor Juhasz, I've named all the enums
in patterndb-int.h and referenced it from all around the data structure
to make their relations more explicit.

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