Upgrade Apache RAT

During local development, I'm regularly annoyed by spurious RAT
failures. While RAT does have some limitations -- for example, because
it never shells out to `git`, it can't ignore untracked files -- newer
versions do have improved support for `.gitignore` files, including the
global gitignore file when present at `~/.config/git/ignore` (the
default location). Additionally, the default exclusions have been
improved, so we no longer need quite as much manual configuration.

Specific changes:

1. Bumped `apache-rat-plugin` directly to 0.17. (In the version of the
   Apache parent POM we are using, there is no property we can override
   to do this.)
2. Removed exclusions that are obsolete or covered by our defaults.
3. Changed `apache-rat:check` to only run in `:httpclient5-parent`,
   instead of separately validating each subproject.
4. Changed `<exclude>` to `<inputExclude>` as the former is now
   deprecated.

Author: rschmitt

pom.xml

@@ -384,25 +384,23 @@
       <plugin>
         <groupId>org.apache.rat</groupId>
         <artifactId>apache-rat-plugin</artifactId>
+        <version>0.17</version>
         <executions>
           <execution>
             <phase>verify</phase>
+            <inherited>false</inherited>
             <goals>
               <goal>check</goal>
             </goals>
           </execution>
         </executions>
-        <configuration>
-          <excludes>
-            <exclude>src/docbkx/resources/**</exclude>
-            <exclude>src/test/resources/*.truststore</exclude>
-            <exclude>src/test/resources/*.serialized</exclude>
-            <exclude>.checkstyle</exclude>
-            <exclude>.externalToolBuilders/**</exclude>
-            <exclude>maven-eclipse.xml</exclude>
-            <exclude>**/serial</exclude>
-            <exclude>**/index.txt</exclude>
-          </excludes>
+        <configuration combine.self="override">
+          <inputExcludes>
+            <inputExclude>**/maven-eclipse.xml</inputExclude>
+            <inputExclude>**/*.pem</inputExclude>
+            <inputExclude>**/index.txt</inputExclude>
+            <inputExclude>**/serial</inputExclude>
+          </inputExcludes>
         </configuration>
       </plugin>
       <plugin>
@@ -538,4 +536,4 @@
     </plugins>
   </reporting>
 
-</project>
+</project>