Verify FWrite() with some example code

[avsa242]

Some code exists for this that hasn't yet been committed to the repo Make sure it can:

• [x] seek to any arbitrary position
• [x] write a block of data to the current position
• [x] enlarge a file (confined to the current sector)
• [x] enlarge a file (confined to the current cluster)
• [x] enlarge a file (allocating cluster(s) as needed)

[avsa242]

• [x] can seek to any arbitrary position

[avsa242]

• [x] can write a block of data to the current position

[avsa242]

• [x] can enlarge a file (confined to the current sector)

[avsa242]

• [x] can enlarge a file (confined to the current cluster)

[avsa242]

all variations above (basically) implemented as of 3464d452e89a43b3b9ba876b68b5549f408ee348