mount: use /dev/.run as an early boot alias for /var/run

mount: use /dev/.run as an early boot alias for /var/run

During early boot, mount a tmpfs to /dev/.run and then bind mount it to
/var/run as soon as /var is available.

This makes it possible for programs involved in early boot to put
runtime data in /dev/.run which later on will show up in /var/run like
any other.

This can be used to solve the early-boot D-Bus problem: D-Bus may start
up with its socket bound to /dev/.run/dbus/system_bus_socket and after
/var it will also be available under the traditional name
/var/run/dbus/system_bus_socket.

This also is intended to be used as a better place for systemd, mount,
mdadm, blkid, plymouth, bootchart and dracut runtime data, which is
currently stored in various places in /dev/.xxx.