Free DOS Memory Block

Frees a memory block that was previously allocated with the
Allocate DOS Memory Block function (Int 31H Fn 0100H).

Call With:

   AX     = 0101H
   DX     = selector of block to be freed

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           0007H   memory control blocks damaged (also
                   returned by DPMI 0.9 hosts).
           0009H   incorrect memory segment specified (also
                   returned by DPMI 0.9 hosts).
           8022H   invalid selector

Notes:

o  All descriptors allocated for the memory block are
   automatically freed by this function, and are no longer
   valid after this function returns.
o  Under DPMI 1.0 hosts, any segment registers which contain
   a selector being freed are zeroed by this function.
o  Refer to the rules for descriptor usage in Appendix D.