BLIOVLSIZ
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  Function : Get current size of overlay pool memory.

  Syntax   : nlValue = BLIOVLSIZ()

  Return   : Overlay Pool Current Size

  This function returns the size of the overlay area as it was actually
  allocated when the program started running. The default size is the most
  recent of the value set at link time with the BLINKER OVERLAY OPSIZE nnn
  command and the /OOnnn setting of the BLINKER environment variable at
  runtime. This size may be modified in one of two ways:

  If the BLINKER OVERLAY PAGEFRAME ON command was used at link time, or
  the /OP1 setting in the BLINKER environment variable is set at runtime,
  then if an EMS pageframe is available and the default OPSIZE is less
  than 64kb, then the size of the overlay area will be expanded to use as
  much of the pageframe as possible. This new size will then be the size
  returned by the BLIOVLSIZ() function.

  If the use of the pageframe is not possible or has been disabled, and
  provided the use of UMBs has not been disabled through the use of the
  BLINKER OVERLAY UMB OFF command at link time or the /OU0 setting of the
  BLINKER environment variable at runtime, then Blinker will attempt to
  allocate a UMB at least as large as the default OPSIZE. If this is
  successful then the size of the overlay area will be expanded to use as
  much of the UMB as possible. This new size will then be the size
  returned by the BLIOVLSIZ() function.

  If neither of these methods is possible, or both have been disabled,
  then the size allocated will be the default OPSIZE.

  Clipper Summer '87 DYNAMIC method note:

  When using the BLINKER OVERLAY DYNAMIC method of overlay allocation,
  this value reflects the exact amount of memory being used by Blinker for
  overlays at the current time. At the start of an application this will
  be zero, and the value will increase until it reaches the operating size
  set by BLINKER OVERLAY OPSIZE <size> and returned by BLIOVLOPS().

  Once it has reached this size it will stabilize for the rest of the
  application until a call to BLIOVLCLR(). When a call to BLIOVLCLR() is
  made, all inactive overlays are discarded and the memory is returned to
  Clipper, so the current size will decrease to the size of just the
  active overlays. In the best case of BLIOVLCLR() being called from the
  root then BLIOVLSIZ() should actually return to zero.

  Example  :
  *  Get the current overlay pool size.
  *
  ? "The current overlay pool size is"
  ?? BLIOVLSIZ(), " bytes"