Fix Extended Clipboard implementation issues

[gujjwal00]

1. According to spec, clipboard text is null terminated and text length includes the null byte. This is how LibVNCServer and other clients encode this. Fixed by 2nd commit https://github.com/LibVNC/libvncserver/commit/7ff0a53d565b6f03380b08b89d05757be82b9c34

2. This is a big one. Many servers (including LibVNCServer) can't properly handle zlib streams created by compress() function of zlib library. When decompressing such streams, inflate() correctly returns Z_STREAM_END, but these servers are only expecting Z_OK. So they end up crashing, while LibVNCserver abruptly disconnects from client.

   This is probably why the original PR #552 was not working. New PR simply suppressed it by adding extra byte, which made inflate() return Z_OK.

Solution:

• LibVNCServer is fixed by 1st commit https://github.com/LibVNC/libvncserver/commit/9593244708e52fb6320b2c0671b569aa18025e41
• TigerVNC is fixed since v1.11
• Submitted fix for UltraVNC (https://github.com/ultravnc/UltraVNC/pull/74)
• Because even after these fixes large number of servers will remain affected, I propose 3rd commit: https://github.com/LibVNC/libvncserver/commit/6adf8e20b72fac84c3a25424c803b79e268e9176. It creates slightly different zlib stream, which does not trigger the bug. It gives us compatibility with all affected servers.

[gujjwal00]

cc @wuhanck

[wuhanck]

it looks ok. it is safer if client dont include \0 in the end.