rst: Don't allocate page for child stack (v2)

When clone-ing kids we can set their stack on current, as
it will anyway be COW-ed later. One thing to note -- we do
need to reserve some space on the stack for glibc's arguments
and retcode allocation. 128 bytes should be enough for 16
pointers while clone has 5 arguments.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
helpers: Create helpers with shared files and fs

They don't change these objects, so can share them
with parent (will be created slightly faster :) ).

The plan is to make them CLONE_VM, but it's not that
easy.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>