Make sure when user is added to qubes group that the group is appended

added -a option to usermod.
This will prevent other groups from being un-subscribed when qubes group is added

archlinux/PKGBUILD.install

@@ -215,7 +215,7 @@ pre_install() {
   # Archlinux bash version has a 'bug' when running su -c, /etc/profile is not loaded because bash consider there is no interactive pty when running 'su - user -c' or something like this.
   # See https://bugs.archlinux.org/task/31831
   useradd --shell /bin/zsh --create-home user
-  usermod --groups qubes user
+  usermod -a --groups qubes user
 }
 
 ## arg 1:  the new package version

debian/qubes-core-agent.preinst

@@ -50,7 +50,7 @@ if [ "$1" = "install" ] ; then
         useradd --user-group --system -M --home /run/tinyproxy --shell /bin/false tinyproxy
     }
     usermod -p '' root
-    usermod -L --groups qubes,sudo user
+    usermod -L -a --groups qubes,sudo user
 
     # --------------------------------------------------------------------------
     # Remove `mesg` from root/.profile?

rpm_spec/core-vm.spec

@@ -116,7 +116,7 @@ groupadd --force --system --gid 98 qubes
 id -u 'user' >/dev/null 2>&1 || {
   useradd --user-group --create-home --shell /bin/bash user
 }
-usermod --groups qubes user
+usermod -a --groups qubes user
 
 if [ "$1" !=  1 ] ; then
 # do this whole %pre thing only when updating for the first time...