[Build] Set all Debian package permissions to 755

755 means the owner can read, write, and execute, and everyone else can just read and execute. I think that's what we want here since without execute permissions others cannot open directories.

Inspired by [this comment on a separate PR](https://github.com/apache/spark/pull/3297#issuecomment-63286730).

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #4277 from nchammas/patch-1 and squashes the following commits:

da77fb0 [Nicholas Chammas] [Build] Set all Debian package permissions to 755

assembly/pom.xml

@@ -39,7 +39,7 @@
     <deb.pkg.name>spark</deb.pkg.name>
     <deb.install.path>/usr/share/spark</deb.install.path>
     <deb.user>root</deb.user>
-    <deb.bin.filemode>744</deb.bin.filemode>
+    <deb.bin.filemode>755</deb.bin.filemode>
   </properties>
 
   <dependencies>
@@ -280,7 +280,7 @@
                         <user>${deb.user}</user>
                         <group>${deb.user}</group>
                         <prefix>${deb.install.path}/conf</prefix>
-                        <filemode>744</filemode>
+                        <filemode>${deb.bin.filemode}</filemode>
                       </mapper>
                     </data>
                     <data>
@@ -302,7 +302,7 @@
                         <user>${deb.user}</user>
                         <group>${deb.user}</group>
                         <prefix>${deb.install.path}/sbin</prefix>
-                        <filemode>744</filemode>
+                        <filemode>${deb.bin.filemode}</filemode>
                       </mapper>
                     </data>
                     <data>
@@ -313,7 +313,7 @@
                         <user>${deb.user}</user>
                         <group>${deb.user}</group>
                         <prefix>${deb.install.path}/python</prefix>
-                        <filemode>744</filemode>
+                        <filemode>${deb.bin.filemode}</filemode>
                       </mapper>
                     </data>
                   </dataSet>