Apparently on FreeBSD one cannot unlink shared mem if it is not created
with write permissions.
Adding IWRITE to the permissions when creating the shared memory is
harmless. It means any process running as the user can either write to
the file or delete it. Deleting was already possible except on FreeBSD.
As for writing, it's needed on FreeBSD for shared memory to be useable
at all, and since processes running as the user can already completely
access all their data, kitty config files, tty pipes, etc. allowing them
to write to the SHM used here doesnt grant any significant new
capability, since they could just write to ssh.conf in the kitty config
folder to get the ssh kitten to do whatever they like.
Fixes#5928
