source4 smdb: Add a post fork hook to the service API

source4 smdb: Add a post fork hook to the service API

Add a post fork hook to the service API this will be called:

 - standard process model
   immediately after the task_init.

- single process model
  immediately after the task_init

- prefork process model, inhibit_pre_fork = true
  immediately after the task_init

- prefork process model, inhibit_pre_fork = false
  after each service worker has forked. It is not run on the service
  master process.

The post fork hook is not called in the standard model if a new process
is forked on a new connection. It is instead called immediately after
the task_init.

The task_init hook has been changed to return an error code. This ensures
the post_fork code is only run if the task_init code completed successfully.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>