pppd: Fix creation of linkpidfile

When pppd is run without "nodetach" or with "updetach", the linkpidfile is
never created. The call to create_linkpidfile() is protected by a check for
linkpidfile[0] but this is only filled in when create_linkpidfile() is called.

This patch changes to code to allways uncondiationally call
create_linkpidfile(), it originated from the Debian project.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

--- a/pppd/main.c
+++ b/pppd/main.c
@@ -773,8 +773,7 @@ detach()
 	/* update pid files if they have been written already */
 	if (pidfilename[0])
 	    create_pidfile(pid);
-	if (linkpidfile[0])
-	    create_linkpidfile(pid);
+	create_linkpidfile(pid);
 	exit(0);		/* parent dies */
     }
     setsid();