Avoid EINTR in readline by repeating poll-call.

[Marthog]

Fixes #402 by repeating the call on EINTR.

[gwenn]

Thanks.
But I am not sure we should loop.

• https://github.com/kkawakam/rustyline/blob/master/src/tty/unix.rs#L417
  Here, we try to make the distinction between a single ESC and an escape sequence.
  I guess we can safely assume that a single ESC was received if an EINTR is raised.
• https://github.com/kkawakam/rustyline/blob/master/src/tty/unix.rs#L705
  Here, we try to find the current cursor position.
  I guess we can safely assume that cursor position is OK if an EINTR is raised.

[bjorn3]

An EINTR can be caused by any signal, including SIGSTOP, SIGCHLD, SIGWINCH or SIGUSR1.

[gwenn]

An EINTR can be caused by any signal, including SIGSTOP, SIGCHLD, SIGWINCH or SIGUSR1.

Indeed!
At least, we should not ignore SIGWINCH.

[gwenn]

See #420.