Improve unnamed_socket diagnostics

[tiif]

Here are some improvements that could be done:

1. Since unnamed_socket read/write is shared between pipe2 and socketpair, the diagnostic for throw_unsup_format shouldn't only mention socketpair:

• miri/src/shims/unix/unnamed_socket.rs

  Line 168 in 00bafbe

  throw_unsup_format!("socketpair read: blocking isn't supported yet");

• miri/src/shims/unix/unnamed_socket.rs

  Line 239 in 00bafbe

  throw_unsup_format!("socketpair write: blocking isn't supported yet");

2. Currently, if O_NONBLOCK flag is used on pipe2, unsupported_error will be thrown. Since we support non-blocking pipe2, we might want to allow this flag.