Implement {,p}{read,write}{,v}().

Add a routine similar to copyinuio() and freebsd32_copyinuio() that
copies in CloudABI's struct iovecs. These are then translated into
FreeBSD format and placed in a 'struct uio', so we can call into the
kern_*() functions.

Obtained from: https://github.com/NuxiNL/freebsd