Compatibility: 3.3+
Expects: AH    67H
BX    desired maximum number of file handles
Returns: AX    error code if CF is set to CY
Info: This function sets the maximum number of file handles that can be
open concurrently by the calling program.

BX can range from 20 to 65535.  If less than 20, 20 is used.

If BX is less than the current maximum (that is, if you want to
shrink the number of handles) and more than BX files are
currently open, then the change will be delayed until there are
no more than the desired number of handles open.

If BX is greater than the current maximum (you want to increase
the number of handles), then DOS must have memory available to
allocate space for the new handles.  Use DOS Fn 4aH (SetBlock) to
shrink the current memory usage.  See Accessing Upper Memory for
a way to put handles in UMB  space.

Notes:   By default, the file handle table is contained in a reserved
portion of each program's PSP.  This call tells DOS to allocate
some memory for an enlarged table.

  This affects only the current application.  The number of
handles available to the system itself is set by the FILES=
command in CONFIG.SYS.  It does not affect child processes.

  Remember that by default, 5 handles are used for standard I/O

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DOS Fn 67H: Set File Handle Count