Follow-up for 2a5364d5

The changes are either applied in the current version or the affected
file is no longer existing.

The normal case was to use the builtin icons. There were two cases
where the icon file was read from disk:

1) /usr/NX/share/images/nxagent.xpm was existing which normally is not
   there

2) case 2 was not true and nxagent.xpm was existing somewhere in the
   PATH (!)

(replace nxagent.xpm by x2goagent.xpm if in x2gp mode)

Scanning the path from the PATH variable for xpm files is kind of
unexpected and dangerous, too (think of automounter triggers or
invalid xpm files).

Also remove the xpm files from the distribution.

This file has been deleted, but references to it in the build remain,
causing `debuild` to fail.

This workaround/hack should not be needed any longer.

debian/patches: Add 2004_enforce-lpthread.patch. On Debian, enforce -lpthread when building nxcomp. This is a nasty work around for a missing symbols issue that needs to get addressed otherwise in later release.

debian/rules: Highly enforce --no-parallel mode while Xserver is still on imake. Can be re-parallelized after autotools conversion.