udev: Fix initializing varlink server from listen fd

udev: Fix initializing varlink server from listen fd

manager_listen_fds() instructs sd_listen_fds_with_names() to unset
the environment which means that when sd_varlink_server_listen_auto()
is called from manager_start_varlink_server(), when it eventually calls
sd_listen_fds_with_names() it will return zero because the environment
has already been unset in manager_listen_fds().

Fix the issue by not using sd_varlink_server_listen_auto() but instead
keeping track of the varlink socket in manager_listen_fds() and returning
it and passing it to manager_start_varlink_server().