Change utils.GenericMain protocol

Currently, GenericMain does a two-staged workflow:

- Check, before forking
- then Exec, after forking

This means we don't have any possibility to treat preparation work
(before the daemon is ready for work) different from the actual work.

The patch adds another PreExec function that is run just before Exec,
and which should ensure that the daemon is ready for serving client
before it returns. Its result is then sent as the third argument to
Exec.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>