add "modified" bit for interupt support

[vitiral]

I had a eureka moment.

This libary could be made to work with volatile data very easily:

• before moving data during defrag, set a modified bit to 0
• whenver data is locked, this bit is set to 1
• every so often, check the bit. If it is 1, start again from the beginning.
• when the data is finished moving atomically and simultaniously check that modified=0 and then set the pointer to it's new location.

Another change is that data can ONLY be moved if its tail will will not overlap with its head (data can never be changed during a move). This is a minor restriction, but a fine one IMO.