unix: add support for libc::readv and libc::writev

[mstoeckl]

readv and writev are vectored versions of read/write which are included in POSIX. (See e.g. man 3p readv, writev .) They could be (and sometimes have been) implemented using a loop and many read or write calls, so their semantics are not unusual.

I was advised to make this issue by the following error message:

test test ... error: unsupported operation: can't call foreign function `readv` on OS `linux`
   --> src/test.rs:125:19
    |
125 |         let ret = libc::readv(src_fd.as_raw_fd(), iovs.as_ptr(), iovs.len() as _);
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function `readv` on OS `linux`
    |
    = help: if this is a basic API commonly used on this target, please report an issue with Miri
    = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases

A workaround for this error: under cfg(miri), replace libc::readv/libc::writev with functions that implement them using libc::read/libc::write directly.