* ./include/vlc/vlc.h, ./src/libvlc.c: added VLC_Error() to the libvlc API.

  * ./include/vlc/vlc.h, ./src/libvlc.c: added VLC_Error() to the libvlc API.
  * ./include/main.h: removed p_vlc->i_status because it was not sufficient
    to represent all the possible states of p_vlc; each part should be tested
    separately upon destruction.
  * ./src/misc/objects.c: fixed a signed/unsigned bug that prevented creation
    of VLC_OBJECT_GENERIC objects.

  * ./src/misc/variables.c: added the VLC_VAR_COMMAND type which is simply a
    variable that stores a function pointer, and calls it when var_Get is
    called for it. The function argument is taken in val.psz_string.
  * ./src/misc/objects.c: vlc_dumpstructure and vlc_liststructure are no longer
    exported to the rest of the program; instead, they're VLC_VAR_COMMAND vars
    ("tree" and "list").
  * ./modules/control/rc/rc.c: moved a few commands to VLC_VAR_COMMAND vars.