DOS Internal - Command.com interface

        Entry:
          AX = 1902h
          ES:DI -> ASCIZ full filename of current batch file, with at least
                   the final filename element uppercased
          DS:DX -> buffer for results

        Return:
          AL = 00h  failed, either
                  (a) final filename element quoted at ES:DI does not match
                      identity of shell batch file quoted as parameter of
                      most recent call of SHELLB command, or
                  (b) no more Program Start Commands available.
          AL= FFh  success, then:
                   memory at DS:[DX+1] onwards filled as:
                   DX+1:        BYTE    count of bytes of PSC
                   DX+2: N      BYTEs   Program Start Command text
                                BYTE    0Dh terminator

        Explanation: COMMAND.COM executes the result of this call in
                     preference to reading a command from a batch file.
                     Thus the batch file does not advance in execution for so
                     long as SHELLB provides PSCs from its workspace.
                     The PSCs are planted in SHELLB workspace by SHELLC, the
                     user menu interface. The final PSC of a sequence is
                     finished with a GOTO COMMON, which causes a loop back
                     in the batch file which called SHELLC so as to execute
                     SHELLC again. The check on batch file name permits PSCs
                     to CALL nested batch files while PSCs are still stacked
                     up for subsequent execution.