rbd: add libkrbd convenience library

rbd: add libkrbd convenience library

Add libkrbd libtool convenience library to provide an interface for
mapping and unmapping rbd images programmatically.  This will be used
by the rbd binary itself and the librbd_fsx testing tool.

libkrbd takes care of the kernel module stuff (common/module.h) and
makes use of libudev to be able to properly wait for block device
creation and deletion and tell which block device got assigned by the
kernel to the newly created mapping.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>