ejabberdctl: support OpenBSD su

OpenBSD has a different than Linux su:
 1. `-c` before username is treated as login class;
 2. it doesn't require `--` as arguments separator.

Without (1) it complains as:

    su: no such login class: exec "$0" "$@"

and without (2):

    -: --: not found

Here, I've added detection of OS via `uname -s` which routes to the
right `su`. I really think that other BSD may need it as well.

Author: catap

ejabberdctl.template

@@ -125,9 +125,16 @@ set_dist_client()
 # run command either directly or via su $INSTALLUSER
 exec_cmd()
 {
-    case $EXEC_CMD in
-        as_install_user) su -s /bin/sh -c 'exec "$0" "$@"' "$INSTALLUSER" -- "$@" ;;
-        as_current_user) "$@" ;;
+    case $EXEC_CMD,$(uname -s) in
+        as_install_user,OpenBSD)
+            su -s /bin/sh "$INSTALLUSER" -c 'exec "$0" "$@"' "$@"
+            ;;
+        as_install_user,*)
+            su -s /bin/sh -c 'exec "$0" "$@"' "$INSTALLUSER" -- "$@"
+            ;;
+        as_current_user,*)
+            "$@"
+            ;;
     esac
 }
 exec_erl()