Log Message:
Update tigervnc to 1.6.0.

Do not update xserver used, still 1.17.2.

Release notes:

TigerVNC 1.6.0 is now available. This release is mostly about fixing
various bugs and polishing existing features, but there are some
notable new things:

* Multi-head support in the Java viewer
* Better certificate handling in the Java viewer, including host
  name verification