Added a new command line parameter (-i or --image=) that allows rbd-fuse...

Added a new command line parameter (-i or --image=) that allows rbd-fuse to specify a single image to be made available within the mount directory. The purpose of this is to allow a single RBD to be "mounted" in user space without opening (and locking) the other images in the pool.

This is accomplished by performing a case-sensitive string compare in enumerate_images() when an image name is specified on the command line. If no image name is specified, all images appear in the mount directory. If a non-existent image name is specified, the mount directory is empty.

Signed-off-by: Stephen F Taylor <steveftaylor@gmail.com>