Bump vm.min_free_kbytes on LVS/cache nodes

Also, introduces new module "vm" and vm::min_free_kbytes() to
set a somewhat-dynamic value based on memory size that's safe
and reasonable, and moves old generic::higher_min_free_kbytes
to vm::higher_min_free_kbytes as well (without change).

Change-Id: I4682a08d967f873bd069aaf0c17ad2b02a0d4d78