os: Properly report failure to link lock file

Stop assuming that a failure to link always means that the file indeed
exists. In case of other failure (e.g., permissions), the user would get an
inconsistent "Can't read lock file" message.

Signed-off-by: Olivier Certner <olce.freedesktop@certner.fr>

os/utils.c

@@ -329,7 +329,7 @@ LockServer(void)
              */
             break;
         }
-        else {
+        else if (errno == EEXIST) {
             /*
              * Read the pid from the existing file
              */
@@ -374,6 +374,12 @@ LockServer(void)
                      LockFile, "\tand start again.");
             }
         }
+        else {
+            unlink(tmp);
+            FatalError
+                ("Linking lock file (%s) in place failed: %s\n",
+                 LockFile, strerror(errno));
+        }
     }
     unlink(tmp);
     if (!haslock)