flowprobe: add api messages to obtain current state

Type: improvement

With this change:
 - add dump/details messages to obtain interfaces for which IPFIX flow
   record generation is enabled;
 - add get message to obtain parameters;
 - add a new message to set parameters with validation present and to
   correspond with get/set naming;
 - add tests for get/set parameters and dump/details interfaces.

Change-Id: I09f6ec990171ac8bcb9d2f5c92629803b8ab6c28
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>