hostnamectl: rework pretty hostname validation (#3985)

Rework 17eb9a9ddb a bit.

Let's make sure we don't clobber the input parameter args[1], following our
coding style to not clobber parameters unless explicitly indicated. (in
particular, as we don't want to have our changes appear in the command line
shown in "ps"...)

No functional change.

src/basic/hostname-util.c

@@ -163,7 +163,6 @@ char* hostname_cleanup(char *s) {
                         *(d++) = *p;
                         dot = false;
                 }
-
         }
 
         if (dot && d > s)