screen: put sockets into $HOME instead of /tmp

on suse, the socket is put into /var/run, which makes more sense
than /tmp.
imo it's even better to put them into home, so other users can not
even see other users sockets.
the only drawback of that is that screen will not work on a read-only
fs (where /var and /tmp are typically on tmpfs and thus writable).
screen: fix breakage when used with run-emulator.sh

when the sabotage rootfs is used in qemu with virtio mount,
new files created get the uid of the qemu user, not the uid of the
user inside qemu. this is usually no problem since you have full
access permissions to everything - however, screen did some uid
comparison checks, which only make sense when the screen sockets
are stored in a public place such as /var/run or /tmp.
in our case, they are stored in $HOME, so any screen there belongs
to us and we do not need to compare uids.