sysusers/generate: bridge 'm' entries to usermod

This tweaks the sysusers.d handling logic so that 'm' entries are
now translated to a series of groupadd + useradd + usermod call.
The last usermod call is the notable change, effectively affecting
the list of secondary groups now.

sysusers.generate-pre.sh

@@ -38,6 +38,7 @@ EOF
 group() {
     group="$1"
     gid="$2"
+
     if [ "$gid" = '-' ]; then
         cat <<-EOF
 	getent group '$group' >/dev/null || groupadd -r '$group' || :
@@ -49,6 +50,17 @@ group() {
     fi
 }
 
+usermod() {
+    user="$1"
+    group="$2"
+
+    cat <<-EOF
+if getent group '$group' >/dev/null; then
+    usermod -a -G '$group' '$user' || :
+fi
+    EOF
+}
+
 parse() {
     while read -r line || [ -n "$line" ] ; do
         { [ "${line:0:1}" = '#' ] || [ "${line:0:1}" = ';' ]; } && continue
@@ -66,7 +78,8 @@ parse() {
                 ;;
             ('m')
                 group "${arr[2]}" "-"
-                user "${arr[1]}" "-" "" "${arr[2]}"
+                user "${arr[1]}" "-" "" "${arr[1]}" "" ""
+                usermod "${arr[1]}" "${arr[2]}"
                 ;;
         esac
     done