rst-malloc: return aligned pointers to sizeof(void *) (v4)

Stas found that if we don't align a pointer,
futex and atomic operations can fail.

v2: don't hard-code the size of void *
v3: add a function to allocate memory without gaps with
    a privious slice. It's used to allocate arrays.
v4: don't change rst_mem_cpos

Cc: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>