pkg: also register rc.installer as shell on install

+POST_INSTALL

@@ -3,6 +3,10 @@ cp /etc/shells /etc/shells.bak
 (grep -v /usr/local/etc/rc.initial /etc/shells.bak; \
     echo /usr/local/etc/rc.initial) > /etc/shells
 rm -f /etc/shells.bak
+cp /etc/shells /etc/shells.bak
+(grep -v /usr/local/etc/rc.installer /etc/shells.bak; \
+    echo /usr/local/etc/rc.installer) > /etc/shells
+rm -f /etc/shells.bak
 
 echo "Registering root shell"
 pw usermod -n root -s /usr/local/etc/rc.initial

+PRE_DEINSTALL

@@ -5,6 +5,9 @@ echo "Updating /etc/shells"
 cp /etc/shells /etc/shells.bak
 (grep -v /usr/local/etc/rc.initial /etc/shells.bak) > /etc/shells
 rm -f /etc/shells.bak
+cp /etc/shells /etc/shells.bak
+(grep -v /usr/local/etc/rc.installer /etc/shells.bak) > /etc/shells
+rm -f /etc/shells.bak
 
 echo "Updating /boot/loader.conf"
 touch /boot/loader.conf