Fixes for the execution functions of services

[gao-yan]

Fix: services: Correctly determine if operations are in-flight

Previously, we kind of abused the return codes of upstart_job_exec(),
systemd_unit_exec() and services_os_action_execute(). For an
asynchronous operation, the return code is supposed to tell whether the
'op' should be freed by the caller. It doesn't exactly indicate whether
the 'op' is in-flight.

For instance, in services_os_action_execute(), if stat() or fork() fail,
operation_finalize() will be called for an asynchronous operation, which
will free the operation and return TRUE if the operation is
non-recurring. But in action_async_helper(), the freed operation
would be considered in-flight and added into inflight_ops list.

[gao-yan]

Fix: systemd: Directly return an error if the connection to System DBus is closed

According to the documentation of dbus_connection_send_with_reply(), if
the connection is disconnected or you try to send Unix file descriptors
on a connection that does not support them, the DBusPendingCall will be
set to NULL. In this case, we can directly return an error instead of
waiting for timeout.

[kgaillot]

Nice catches, thanks