When encountering an error writing to stdout or stderr, rbssh will try

again until successful, assuming that the pipe was just blocked.

However, if the pipe is closed, it just spins forever. This can happen

if a caller closes stdout/stderr before all output has been exhausted.
This change fixes that to listen for a closed pipe, and just gives up on

writing.
It also removes the new addition of a sleep call, which would have made

sense when polling, but not when blocking on output. It just caused

rbssh to be slower.

All unit tests passed.
Ran through a reproduction case that resulted in an infinite loop due

to a closed pipe. Verified it's been fixed.