libceph: add, don't set data for a message

Change the names of the functions that put data on a pagelist to
reflect that we're adding to whatever's already there rather than
just setting it to the one thing.  Currently only one data item is
ever added to a message, but that's about to change.

This resolves:
    http://tracker.ceph.com/issues/2770



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>