Bug#64906: libc6 breaks upgrade from 1.3.1 [+fix]
In libc6 postinst, updatercd() is called to install devpts stuff in /etc/rcS.d
It does a 'cd /etc/rc$1.d' ($1="S") without checking if that dir already
This breaks upgrades from 1.3.1 in which /etc/rcS.d doesn't exist yet.
libc6 postinst terminates unsuccessfully and apt refuses to do anything else.
/etc/rcS.d is provided by the sysvinit package, but that pre-depends on libc6
which doesn't help things.
Fix is super-easy: 'mkdir -p /etc/rc$1.d' before 'cd /etc/rc$1.d'