support/scripts/mkusers: fix UID/GID confusion

Commit 41ea61d59c3e (support/scripts/mkusers: allow option for system
uid/gid) confused GID and UID variables: the GID limits were used to
create UIDs.

Fix that.

Note that this fixes a shellcheck error; although there are many more
shellcheck errors, these fixes are semantically a bug that need to be
fixed separately from the coding style issues reported by shellcheck.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Norbert Lange <nolange79@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/scripts/mkusers

@@ -324,9 +324,9 @@ add_one_user() {
 
     # Generate a new UID if needed
     if [ ${uid} -eq ${AUTO_USER_ID} ]; then
-        uid="$( generate_uid "${username}" $FIRST_USER_GID $LAST_USER_GID )"
+        uid="$( generate_uid "${username}" $FIRST_USER_UID $LAST_USER_UID )"
     elif [ ${uid} -eq ${AUTO_SYSTEM_ID} ]; then
-        uid="$( generate_uid "${username}" $FIRST_SYSTEM_GID $LAST_SYSTEM_GID )"
+        uid="$( generate_uid "${username}" $FIRST_SYSTEM_UID $LAST_SYSTEM_UID )"
     fi
 
     # Remove any previous instance of this user