debian/libnx-xinerama1.*: move Xinerama dir back to nx-x11-common. Only delete known files. Fixes RPM build failures.

libnx-xinerama1: also create libXinerama symlink in libnx-xinerama1.postinst.postinst (and remove in libnx-xinerama1.postinst.prerm.)

Due to the nx-x11-common package being a noarch/allarch package,
creating the symlink in nx-libs' Makefile will lead to the symlink
referencing the "default" architecture dpkg uses for building
noarch/allarch packages.

Incidentally, this worked fine for Debian, as amd64 seems to be the
default architecture. On Ubuntu, however, the default architecture up to
Vivid (15.04) was i386. For those builds, the symlink pointed to the 32
bit library of libNX_Xinerama.so.1 -- essentially breaking this feature.

Move the symlink creation to the arch-sensitive libnx-xinerama1 package.
The postinst and prerm scriptlets will work fine, unless someone
installs the i386 package version *after* the amd64 version. Given that
we already create symlinks to libNX_X11 and friends using that method,
no new regression is introduced. Strictly speaking that's a bug, but
we'll hopefully clean that up later...

  * Debian/Ubuntu packaging:
    + Fully rework the way nx-libs gets packaged for Debian/Ubuntu.
    + Split up libnx-x11 into individual packages.
    + Provide dbg:packages for each bin:package containing binaries.
    + Use Makefile logic to install files into DESTDIR.
    + Provide dev:packages for each lib:package individually.
    + Provide nx-x11proto-*-dev packages for all libnx-* libraries.
    + Install _all_ library files (*.so*) to /usr/lib/<triplet>/, so
      no extra settings of LD_LIBRARY_PATH is necessary.

Provide temporary workaround for installation on Debian 64bit with multiarch support (via libnx-x11.postinst and .prerm).