We have a function pair rrd_import() / rrd_export() now, which do
wildly different things even though they've been used for a single
purpose:

(1) rrd_export() writes a string, appends it to the config.xml

(2) rrd_import() takes a parsed config portion, but not as an
    argument, rather as a global config element

(3) There was also an older backup clue which exported files, but
    simply packed them up for restore after boot

If we can get the system to unify the usage we'll end up with a
neat way of backing, restoring and migrating RRD data, although
I'm not sure if this can be fully compatible with the old files.

(cherry picked from commit c7a488dc)