os: Fix -logfile when used with -displayfd

Trivial way to reproduce the bug:

$ Xorg -logfile /tmp/mylog -config /etc/xpra/xorg.conf -displayfd 2

The server then moans:

Failed to rename log file "/tmp/mylog" to "/tmp/mylog": No such file or directory

And the log file is created but immediately renamed to "/tmp/mylog.old".
This is caused by the changes to the log file handling introduced by
this commit:

https://cgit.freedesktop.org/xorg/xserver/commit/?id=edcb6426f20c3be5dd5f50b76a686754aef2f64e

To fix this, only rename the logfile if the log filename contains the
magic substitution string "%s".

Signed-off-by: Antoine Martin <antoine@nagafix.co.uk>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>

os/log.c

@@ -296,7 +296,7 @@ LogInit(const char *fname, const char *backup)
 void
 LogSetDisplay(void)
 {
-    if (saved_log_fname) {
+    if (saved_log_fname && strstr(saved_log_fname, "%s")) {
         char *logFileName;
 
         logFileName = LogFilePrep(saved_log_fname, saved_log_backup, display);