Miscellaneous improvements to activation/item updater

This commit slightly enhances and improves the item updater and the
activation process, addressing a few issues that were raised in code
review of openbmc/openbmc#1756

1.) Service file booleans used in activation are reset to false, to
avoid a BMC-flavored issue in the vein of openbmc/openbmc#1984

2.) systemd constants used in multiple places are moved to configure.ac

3.) Activation objects are unsubscribed from systemd signals at the end
of that process.

Resolves openbmc/openbmc#2031

Change-Id: I573645b16bca28ac6bf3b173d5b4845205224e07
Signed-off-by: Michael Tritz <mtritz@us.ibm.com>
diff --git a/activation.hpp b/activation.hpp
index 41d055e..1e91759 100644
--- a/activation.hpp
+++ b/activation.hpp
@@ -173,6 +173,16 @@
          */
         void subscribeToSystemdSignals();
 
+        /**
+         * @brief unsubscribe from the systemd signals
+         *
+         * systemd signals are only of interest during the activation process.
+         * Once complete, we want to unsubscribe to avoid unnecessary calls of
+         * unitStateChange().
+         *
+         */
+        void unsubscribeFromSystemdSignals();
+
         /** @brief Persistent sdbusplus DBus bus connection */
         sdbusplus::bus::bus& bus;