The memory in question is allocated and then remaped into
restorer with one call to mremap. Thus, to use the mem we
need to provide to users API for

* alloc memory
* get current allocation pointer
* convert current pointer into remapped one
* the remap itself

This would help combinding lots of small allocations such
as tcp sockets to unrepair, siginfos and posix timers.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>