make illumos uiocopy use vn_io_fault_uiomove

uiocopy() is currently unused, its purpose is copy data from a uio
without modifying the uio. It was in use before the vn_io_fault support
was added to ZFS, at which point our code diverged from the illumos code
a little bit. Because ZFS is the only (potential) user of the function
we are free to modify it to better suit ZFS needs.

The intention behind this change is to remove the differences introduced
earlier in zfs_write().

While here, re-implement uioskip() using uiomove() with
uio_segflg == UIO_NOCOPY.
The story of uioskip is the same as with uiocopy.

Reviewed by: mav
MFC after: 1 week