Disable exclusive access mode during close.

When closing the slave side of a pseudo terminal, the exclusive access
mode will persists on the master side. The result is that re-opening the
slave side will fail with EBUSY, unless the process has root priviliges.
To workaround this problem, we already introduced an option that enables
better compatibility with pseudo terminals. See commmit
fab606b00a44ea2114a4029ad09b70c66c3049f7 for details.

In my development environment, I always have this option enabled. But
occasionally I also need to test release builds. And then I usually end
up with inaccessible pty's again, because the pty support is disabled by
default for release build.

This problem can easily be avoided by disabling the exclusive access
mode, just before closing the file descriptor.

src/serial_posix.c

@@ -205,6 +205,11 @@ serial_close (serial_t *device)
 		return -1;
 	}
 
+#ifndef ENABLE_PTY
+	// Disable exclusive access mode.
+	ioctl (device->fd, TIOCNXCL, NULL);
+#endif
+
 	// Close the device.
 	if (close (device->fd) != 0) {
 		SYSERROR (device->context, errno);