* Make readv/writev implementations update struct uio
This can simplify partial result handling.
* change the error number on the overflow from EOVERFLOW to EINVAL
to match NetBSD
* add a commented out uio_offset field. I used "#if 0" here as
C comments can't nest.
* add a few helper functions
Note on uio_copyfrom/uio_copyto:
although i'm not quite happy with the "offset" functionality,
it's necessary to simplify the adaptation of some drivers like
drivers/serial/serial.c, which (ab)uses the user-supplied buffer
as a line-buffer.
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