Add function to remap a structure

Add function to remap a structure

This function will be used to map two structures from two
different sysstat versions.
It will thus be possible to add new fields, e.g., in structures
containing statistics and still be possible for another sysstat version
to read its contents properly.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>