tree-wide: correct cases where return log_{error,warning} is used without value

In various cases, we would say 'return log_warning()' or 'return log_error()'. Those
functions return 0 if no error is passed in. For log_warning or log_error this doesn't
make sense, and we generally want to propagate the error. In the few cases where
the error should be ignored, I think it's better to split it in two, and call 'return 0'
on a separate line.

src/systemctl/systemctl.c

@@ -2086,8 +2086,10 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
         if (streq(key, "systemd.unit")) {
                 if (proc_cmdline_value_missing(key, value))
                         return 0;
-                if (!unit_name_is_valid(value, UNIT_NAME_PLAIN|UNIT_NAME_INSTANCE))
-                        return log_warning("Unit name specified on %s= is not valid, ignoring: %s", key, value);
+                if (!unit_name_is_valid(value, UNIT_NAME_PLAIN|UNIT_NAME_INSTANCE)) {
+                        log_warning("Unit name specified on %s= is not valid, ignoring: %s", key, value);
+                        return 0;
+                }
 
                 return free_and_strdup_warn(ret, key);