Changes implemented in DeviceFS by JSR:

* Optimise CallBuffMan to call with LDMIA
* Fix bug in create buffer for TX to quote internal handle, not external one.
* Fix bug is fs_get where 2nd time round loop would try args_eof with a duff file handle.
* fs_get calls MonitorRX if goes round loop more than once.
* fs_put calls MonitorTX if goes round loop more than once.
* Fix bug in gbpb_get to not ignore checkescape.
* Add MonitorTX to gbpb_put and MonitorRX to gbpb_get.
* Fix bug in gbpb_put to not ignore checkescape.
* Add MonitorEOF flag to fs_get and fs_gbpb read.
* Fix bug in gbpb_put where escape got swallowed by purgebuffer.
* Ignore rather than return error on Args_SetEXT. This is to ensure
compatibility with C file I/O.

To the interface:

Added flag:
ParentFlag_MonitorTransfers bit 2

When this flag is set DeviceFS monitors transfers using the new reason codes
MonitorTX (12) and MonitorRX (13). r2 = internal handle. Return an error if
a transfer is in difficulties.

Added flag:
ParentFlag_MonitorEOF bit 3

When this flag is set then DeviceFS will monitor EOF during a read
operation. For example Serial and Parallel input don't want this set (hang,
waiting for input), but Tape input does want it set (file mark reached).
