(*) O_APPEND is *not* functional yet, this is just a hack for write-only
streams. Currently O_APPEND does not reposition the cursor at the end of
the file before every write.
There is no evidence that BFile_Create() keeps the address and writes
to it later on, but I'd rather be overly careful than have to debug a
stack corruption problem in half a year :)
This helped locate some bugs:
* read() could read past EOF due to BFile_Read() allowing you to read up
until the end of the last sector, beyond the file size
* pread() did not restore the file offset because the negative seek at
the end is not relative (that was the CASIOWIN fs API), so pread()
could not actually be written without knowing the current position
* lseek() would clamp you to EOF but still return its out-of-bounds
arguments, as a direct result of BFile_Seek() doing that
Benefits:
* Made pread() a generic function
