Port _FileDescriptorStreamsNonBlocking to use poll()

select() has a limit of FD_SETSIZE file descriptors. If you run repo
sync -j500 you'll pretty quickly hit this limit and get "file descriptor
out of range for select" errors. poll() has no such limit.

Change-Id: I21f350e472bda1db03dcbcc437645c23dbc7a901
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/248852


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Theodore Dubois <tbodt@google.com>