Fix "No passwd entry for user '-'"

After the update of the shadow package from version 4.1.5.1 to 4.12.3,
"su" changed its interpretation of the pattern

    su - -

so that the second "-" is now interpreted as the username which leads
to this error:

    starting /run/mariux/mxvip.bla.cfg ..
    starting /run/mariux/mxservice.bla.cfg ..
    No passwd entry for user '-'

Fix this by coding the interpretation of "-" as "root", which is
specific to mxstartup anyway, explicitly.

mxservicectl

@@ -19,6 +19,8 @@ function mxsrv_start_one() {
 
    . ${cfg}
 
+   test "$MX_SRV_USER" = "-" && MX_SRV_USER="root"
+
    case "${MX_SRV_SCRIPT}" in
        *.service)
            systemctl start "${MX_SRV_SCRIPT}" || true
@@ -41,6 +43,8 @@ function mxsrv_stop_one() {
 
    . ${cfg}
 
+   test "$MX_SRV_USER" = "-" && MX_SRV_USER="root"
+
    case "${MX_SRV_SCRIPT}" in
        *.service)
            systemctl stop "${MX_SRV_SCRIPT}" || true