Remote shell server: add SIGIO handler earlier

Before this there used to be a short window of time between adding O_ASYNC and adding the signal handler where a client could write on the channel, resulting in a SIGIO that then terminates the remote shell server. I've seen one instance of this on jenkins: https://jenkins.tails.boum.org/job/test_Tails_ISO_20390-fix-fragile-step/7/

Closes: #20404 (closed)