MXS-5160: Silence installation output

The systemctl enable and disable commands printed output to the standard
error which caused messages to appear during package installation and
removal.

Also added missing redirections into a few commands and made sure that
there are spaces between all of the the > operators.

etc/postinst.in

@@ -42,7 +42,7 @@ else
 fi
 
 # Only copy the service files if the systemd folder and systemctl executable are found
-if [ -f @CMAKE_INSTALL_PREFIX@/@MAXSCALE_SHAREDIR@/maxscale.service ] && command -v systemctl >/dev/null
+if [ -f @CMAKE_INSTALL_PREFIX@/@MAXSCALE_SHAREDIR@/maxscale.service ] && command -v systemctl > /dev/null
 then
     if [ -d "/lib/systemd/system"  ]
     then
@@ -64,7 +64,7 @@ then
     fi
 
     mkdir -p /etc/systemd/system/maxscale.service.d
-    systemctl enable maxscale.service > /dev/null
+    systemctl enable maxscale.service >& /dev/null
 
     systemctl is-active maxscale.service --quiet > /dev/null && systemctl restart maxscale.service > /dev/null
 else

etc/prerm.in

@@ -7,7 +7,7 @@ if [ "$1" = "0" ] || [ "$1" = "remove" ]
 then
     if [ -f /etc/init.d/maxscale ]
     then
-        /etc/init.d/maxscale stop
+        /etc/init.d/maxscale stop > /dev/null
         rm /etc/init.d/maxscale
     fi
 
@@ -18,16 +18,16 @@ then
 
     if [ -f /usr/lib/systemd/system/maxscale.service ]
     then
-        systemctl stop maxscale.service
-        systemctl disable maxscale.service > /dev/null
+        systemctl stop maxscale.service > /dev/null
+        systemctl disable maxscale.service >& /dev/null
         rm /usr/lib/systemd/system/maxscale.service
-        systemctl daemon-reload
+        systemctl daemon-reload > /dev/null
     elif [ -f /lib/systemd/system/maxscale.service ]
     then
-        systemctl stop maxscale.service
-        systemctl disable maxscale.service > /dev/null
+        systemctl stop maxscale.service > /dev/null
+        systemctl disable maxscale.service >& /dev/null
         rm /lib/systemd/system/maxscale.service
-        systemctl daemon-reload
+        systemctl daemon-reload > /dev/null
     fi
 
     if [ -f /etc/logrotate.d/maxscale_logrotate ]