Use corresponding syscalls for {p,}{read,write}{v,}
Currently, only read and write, out of read/pread/readv/preadv/preadv2 and write/pwrite/writev/pwritev/pwritev2, are part of the platform abstraction trait. On Linux, adding missing syscall wrappers for this would be relatively easy, and Redox will likely get support for them soon (only preadv2 and pwritev2 as they are the superset of all the other ones).