Misc fixes
- Fix readers which were sleeping in BGET being left blocked when the writer closes the file (discovered while investigating https://www.riscosopen.org/tracker/tickets/480)
- Fix a register corruption bug in fsfile_ReadInfo which would cause it to fail with a "file not found" response if the file is open for writing (instead of the intended behaviour of blocking until the writer is closed, and then returning up-to-date info) (this bug is probably the reason why https://www.riscosopen.org/tracker/tickets/480 wasn't discovered 30-odd years ago)
- Make some effort to protect the dynamic area from unwanted access
Tested in conjunction with FileSwitch!14 (merged) building & running a ROM, along with misc PipeFS using things, and a bit of testing of a 26bit build on RISC OS 3.7.