Warn explicitly about configuration file updates

The configuration file will never be overwritten automatically.
Therefore, changes need to be applied by the user manually.

CMakeLists.txt

@@ -100,6 +100,6 @@ if(USE_APPARMOR)
 endif()
 
 install(TARGETS ${PROJECT_NAME} DESTINATION "sbin")
-install(FILES README.md main.cf.ex DESTINATION "${DOC_DIR}")
+install(FILES README.md README_UPGRADE.md main.cf.ex DESTINATION "${DOC_DIR}")
 install(SCRIPT "${CMAKE_CURRENT_BINARY_DIR}/postinstall.cmake")
 

README_UPGRADE.md

@@ -0,0 +1,15 @@
+PostSRSd Upgrade Note
+=====================
+
+/etc/default/postsrsd
+---------------------
+
+The shipped configuration file has changed from previous versions.
+However, the installer will not automatically overwrite existing
+configuration files to prevent data loss.
+
+Please review the changes after the installation and update your
+configuration file accordingly. Note in particular, that most options
+may no longer remain commented out, since it was cumbersome to maintain 
+all the default values in the various startup scripts.
+

postinstall.cmake.in

@@ -15,8 +15,20 @@ if(CHROOT_DIR AND NOT EXISTS "$ENV{DESTDIR}${CHROOT_DIR}")
 	file(MAKE_DIRECTORY "$ENV{DESTDIR}${CHROOT_DIR}")
 endif()
 
-if(INIT_FLAVOR AND NOT EXISTS "$ENV{DESTDIR}${CONFIG_DIR}/@PROJECT_NAME@")
-	file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/@PROJECT_NAME@.default" DESTINATION "${CONFIG_DIR}" RENAME "@PROJECT_NAME@")
+if(INIT_FLAVOR)
+	if (NOT EXISTS "$ENV{DESTDIR}${CONFIG_DIR}/@PROJECT_NAME@")
+		file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/@PROJECT_NAME@.default" DESTINATION "${CONFIG_DIR}" RENAME "@PROJECT_NAME@")
+	else()
+		file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/@PROJECT_NAME@.default" DESTINATION "${CONFIG_DIR}" RENAME "@PROJECT_NAME@.new")
+		message(STATUS "")
+		message(STATUS "*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*")
+		message(STATUS "*!* ${CONFIG_DIR}/@PROJECT_NAME@ will NOT be overwritten!")
+		message(STATUS "*!* Please note the changes from @PROJECT_NAME@.new in the same folder and")
+		message(STATUS "*!* update your configuration accordinly.")
+		message(STATUS "*!* See also README_UPGRADE.md for details")
+		message(STATUS "*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*!*")
+		message(STATUS "")
+	endif()
 endif()
 
 if(INIT_FLAVOR STREQUAL "sysv-lsb")