Fix "No passwd entry for user '-'"

[donald]

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.

[donald]

Maybe we should move this to mxtools? Much overhead for a small fix in a perl script.

[wwwutz]

Maybe we should move this to mxtools? Much overhead for a small fix in a perl script.

Hmm, mxstartups could run anywhere, it's not MX dependend, so I think keeping it here as single repo is ok.