ossec-execd should support more granular controls around uid/gid of self and children

[drawks]

As mentioned in the discussion in issue #207 execd is run as root as doesn't drop privileges since it may be used typically for executing active response commands which typically required root permissions. It would be a nice feature if execd could optionally drop root privileges if they aren't required by the end user's configuration.

Likewise if exed is run with root privileges it would be nice if the command portion of the active response configuration allowed the uid/gid of spawned processes to be specified. This would allow for the normal practice of least required privileges to be extended to child processes.