kernel: restructure readi/writei to save memory

kernel: restructure readi/writei to save memory

This saves us 256 bytes on Z80 mostly by removing the amount of 32bit maths
SDCC has the opportunity to **** up.

We introduce a helper (umove) which adjusts all the pointers and offsets
for a given file and use that where we can. We also restructure a bit so we
can use the udata.u_done value in readi/writei

This has some minor consequences:
Callers need to check u_done for completion not u_count
Char and Socket handlers don't affect the offset unless they choose to (which
is saner). Devsys has been modified accordingly.

Handle with care.