librados: add exec to the c object operations api

librados: add exec to the c object operations api

Add two versions: one that allocates a buffer of the appropriate
length for the user, but relies on the user to free it, and one that
uses a user-supplied buffer but may fail if it is too short.

Reuse the bufferlist -> buffer conversion context added for reads into
the user supplied buffer.

The librados-allocated buffer can be handled just like librados
allocated buffers used by the various command functions, so just reuse
do_out_buffer() for them.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>