Revert "Switch from using libNX_X11's deprecated XKeycodeToKeysym() function to using XGetKeyboardMapping()."

This reverts commit efc0dae0.

Recent test revealed keyboard hangs on high latency connections. These
hangs are not happening in 3.5.0 releases if the nx-libs.

The commit above is responsible for them so we take that back.

Interestingly nxcomp has special treatment for XGetKeyboardMapping()
so it should normally speed up things but it results in the opposite
for us. Needs further examination.

This fixes ArcticaProject/nx-libs#450

Fixes: ArcticaProject/nx-libs#432

debian/copyright: Update copyright attributions (re-add nx-X11/programs/Xserver/hw/nxagent/NXpicturestr_PictSolidFill.h).

Attributes GH PR #437: https://github.com/ArcticaProject/nx-libs/pull/437

Xserver/Imakefile: Drop redundant library path option -L../../../nx-X11/exports/lib when linking nxagent. As -L../../exports/lib is already present and points to the same location.

Xserver/Imakefile: Change order of linker options, so that self-built components get mentioned first in the nxagent linker command.

Attributes GH PR #436: https://github.com/ArcticaProject/nx-libs/pull/436

 Fixes ArcticaProject/nx-libs#370.

 Fixes ArcticaProject/nx-libs#383.

 This reverts commit 6c8c9502.

 Fixes ArcticaProject/nx-libs#433.

 This patch is still a candidate for being re-implemented without extending the
 _PictSolidFill struct, but for now, we'll have to re-introduce commit db8705
 for the sake of having fonts readable after resumption of remote NX sessions.

Attributes GH PR #419: https://github.com/ArcticaProject/nx-libs/pull/419

Xserver/os/connection.c: If -displayfd and an explicit display port number are given, use the explicit display number as a starting point for auto-detecting the next available display number.

Xserver/os/connection.c: Accompany display number with some human-readable (machine-parseable) text when -displayfd is set to STDERR.

Attributes GH PR #431: https://github.com/ArcticaProject/nx-libs/pull/431

"nx-X11 Agent" looks and sounds ugly.

Attributes GH PR #434: https://github.com/ArcticaProject/nx-libs/pull/434

nx-libs.spec: sync up RPM builds after Xext proto change in a51cb945.

debian/{rules,nx-x11proto-xext-dev}: More Xext header files belong int nx-x11proto-xext-dev (instead of removing them in debian/rules).

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

Attributes GH PR #421: https://github.com/ArcticaProject/nx-libs/pull/421

 Fixes ArcticaProject/nx-libs#80

Attributes GH PR #428: https://github.com/ArcticaProject/nx-libs/pull/428