Properly enable/disable Xinerama support in nxagent via cmdline options provider in Xserver/os/utils.c.

  * Xinerama defaults to enabled, using RRXineramaExtension
    (i.e. cmdline option: +rrxinerama).
  * Cmdline option -rrxinerama disables Xinerama support entirely.
  * Cmdline option +xinerama switches from RRXineramaExtension to
    PanoramiXExtension.
  * Cmdline option -xinerama is actually non-functional (i.e. the
    default).
  * If the nx/nx option "xinerama" is handed over to nxagent,
    then Xinerama is only switched on if that given option enables it
    (value: 1) _and_ if either of the Xinerama extensions (PanoramiX
    or RRXinerama) has been initialized (via cmdline options above).

 Fixes ArcticaProject/nx-libs#117.
 Fixes ArcticaProject/nx-libs#134 (makes flawed PR#134 obsolete).