service: rework how we release resources

service: rework how we release resources

Let's normalize how we release service resources, i.e. the three types
of fds we maintain for each service:

1. the fdstore
2. the socket fd for per-connection socket activated services
3. stdin/stdout/stderr

The generic service_release_resources() hook now calls into
service_release_fd_store() + service_close_socket_fd()
service_release_stdio_fd() one after the other, releasing them all for
the generic "release_resources" infra of the unit lifecycle.

We do no longer close the socket fd from service_set_state(), moving
this exclusively into service_release_resources(), so that all fds are
closed the same way.