Get Vendor-Specific API Entry Point

Returns an address which can be called to use host-specific
extensions to the standard set of DPMI functions. This
function is available only in protected mode.

Call With:

   AX     = 168AH
   DS:(E)SI= selector:offset of ASCIIZ (null-terminated)
           string identifying the DPMI host vendor

Returns:

   if function successful
   AL     = 0
   ES:(E)DI= extended API entry point

   and DS, FS, GS, EAX, EBX, ECX, EDX, ESI, and EBP are
           preserved.

   if function unsuccessful
   AL     = unchanged (8AH)

Notes:

o  The ASCIIZ string specifies a host vendor name or some
   other unique identifier to obtain a specific extension
   entry point.  The string comparison used to look up the
   API entry point is case-sensitive.
o  Clients must use a FAR CALL to reach the extended API
   entry point.
o  All extended API parameters are specified by the vendor.
o  DPMI 1.0 clients should use this function in preference
   to Int 31H Fn 0A00H.  This method of API extension
   is preferable to the Int 31H extension as it avoids the
   creation of a long (and consequently slow) chain of Int
   31H handlers which would slow down time-critical DPMI
   functions. Note that although this function was not
   documented for DPMI 0.9, it will work under any DPMI 0.9
   host.