Don't sometimes hang just after launching ssh

sshd has a bug in which the sometimes it may get stuck trying to read from the client
even though the child process has already exited. This is visible at
https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/serverloop.c?annotate=1.226 line 274:
once the child is waited on, `child_terminated` is reset to 0, which causes it to use an infinite timeout 
in the select there.

This workaround causes mosh to disconnect from the server, thereby allowing sshd finish.