MINOR: startup: change session/process group settings

MINOR: startup: change session/process group settings

Change the way the process groups are set. Indeed setsid() was called
for every processes which caused the worker to have a different process
group than the master.

This patch behave in a better way:

- In daemon mode only, each child do a setsid()
- In master worker + daemon mode, the setsid() is done in the master before
forking the children
- In any foreground mode, we don't do a setsid()

Could be backported in 1.8 but the master-worker mode is mostly used
with systemd which rely on cgroups so that won't affect much people.