udevd: merge manager_new() and manager_listen() again
Now that listen_fds() have been split out, we can safely move the allocation
of the manager object after doing the forking (the fork is done to notify legcay
init-systems that the fds are ready).
Subsequently, we can merge manager_listen() back into managre_new().
This entails a minor behaviour change: the application of permissions to
static device nodes now happens after the fork (but still before notifying
systemd about being ready).