unix-socket: introduce API to add commands and tasks

unix-socket: introduce API to add commands and tasks

This patch transforms the unix socket into a flexible system to
add commands (triggered by user) and taks (run periodically).
It introduces two functions UnixManagerRegisterCommand and
UnixManagerRegisterBackroundTask to registed commands and tasks.

Other part of Suricata can then declare a new command via a simple
call of the function. In the case of a command the caller is
responsible of building the answer message using Jansson API. The
sending of the message is made by unix manager code.