cgmanager.c: mount_tmpfs_at() uses mode=0755 mount option

I also added a minor fix so that path is used in the debug message, not CGDIR.

This fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=766565

I think this problem is not apparent on Upstart (i.e. Ubuntu) systems because Upstart uses mountall, and recent versions of mountall (trusty+ IIRC) mount /sys/fs/cgroup for cgmanager.