Avoid returning EINTR in PosixRenderer::move_cursor_at_leftmost

[Marthog]

Hello,
I could trace back a weird termination in nushell on linux to an EINTR returned by a poll call in rustyline.

The issue occurs when I run qterminal -e nu, but is timing dependent and may be not reproducible on other devices. Even small changes like additional logging changed the behavior during debugging.

There are multiple potential fixes:

1. Repeat the call on EINTR.
2. use sigaction to repeat all interrupted syscalls. This may prevent multiple similar issues, but unfortunately it is a program-wide change.

I will send a pull request soon, with a fix for option 1.