lowmem: add compatibility code

This is a patch that will reintroduce the old sysctl lowmem variables as read
only values in /proc/sys/vm. Some parts of the userland (at least libosso) are
reading these and it would probably greatly ease the transition to the new
scheme with memory limits in pages. We can revert this ugly patch when userland
has moved to the new scheme.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>