debian/control: Pull-over latest changes on these files from the official Debian package. Various improvements several package descriptions.

 Fixes ArcticaProject/nx-libs#80

debian/control: B-D alternatively on libxfont1-dev or libxfont-dev. Fix FTBFS on Debian stretch which has libxfont2-dev as default now.

 Fixes ArcticaProject/nx-libs#295.

 Fixes ArcticaProject/nx-libs#276.

debian/control: Add some hard and exact-versioned dependencies for our bin:packages. Assure that no old share lib packages with the same set of symbols remain on the system when upgrading.

debian/control: Drop various not-used-anymore libNX_* related packages. Add B:/R: to libnx-x11-{6,dev,6-dbg) to uninstall those package when upgrading to nx-libs 3.6.x.

 The x2goagent package is being replaced by an x2goserver bin:package
 named x2goserver-x2goagent.

 This x2goserver-x2goagent bin:package will be maintained in the X2Go
 Project's namespace and basically wraps around nxagent.

 Furthermore, declare Arctica Project as upstream source for nx-libs.

fix wrong bin:package for Xext proto headers: Move dpmsstr.h and securstr.h from Xext devel package into proto header package.

library clean-up: Don't build libNX_Xinerama anymore. Use system's libXinerama shared library. (Fixes ArcticaProject/nx-libs#49).

 This commit goes along with a patch from Ulrich Sibiller who managed to
 move the Xinerama awareness for NX sessions into the Xserver code. This
 makes Xinerama support for NX in libNX_Xinerama.so obsolete.

 Fixes ArcticaProject/nx-libs#49

library clean-up: Don't build libNX_Xtst anymore. Use system's libXtst shared library. (Fixes ArcticaProject/nx-libs#32).

Cherry-picked from 3.5.0.x branch.