Support for relocatable RPMs

Save the value of `RPM_INSTALL_PREFIX` into application's sysconfig file.

Upon application start up, look for this value, else use the `chdir` value
supplied via sbt templates.

src/main/resources/com/typesafe/sbt/packager/archetypes/java_server/rpm/preinst-template

@@ -8,4 +8,7 @@ then
 
     addGroup ${{daemon_group}} "${{daemon_group_gid}}"
     addUser ${{daemon_user}} "${{daemon_user_uid}}" ${{daemon_group}} "${{app_name}} user-daemon" "${{daemon_shell}}"
-fi
+fi
+
+[ -e /etc/sysconfig/${{app_name}} ] && sed -i 's/PACKAGE_PREFIX\=.*//g' /etc/sysconfig/${{app_name}}
+[ -n "$RPM_INSTALL_PREFIX" ] && echo "PACKAGE_PREFIX=${RPM_INSTALL_PREFIX}" >> /etc/sysconfig/${{app_name}}

src/main/resources/com/typesafe/sbt/packager/archetypes/java_server/systemloader/systemv/start-rpm-template

@@ -36,9 +36,10 @@
 # This order means system config appends/overrides package config
 [ -e /etc/sysconfig/${{app_name}} ] && . /etc/sysconfig/${{app_name}}
 
-cd ${{chdir}}
+INSTALL_DIR="${{chdir}}"
+[ -n "${PACKAGE_PREFIX}" ] && INSTALL_DIR="${PACKAGE_PREFIX}/${{app_name}}"
 
-exec="${{chdir}}/bin/${{exec}}"
+exec="$INSTALL_DIR/bin/${{exec}}"
 prog="${{app_name}}"
 lockfile="/var/lock/subsys/${{app_name}}"