tools/ceph-dencoder: build dencoders as plugins

tools/ceph-dencoder: build dencoders as plugins

to reduce the memory footprint when linking ceph-dencoder.

* src/tools/ceph-dencoder:
  * build dencoders as shared libraries named with the prefix of
    "den-mod-". so ceph-dencoder can find them
  * install dencoders into $prefix/lib/ceph/denc, so ceph-dencoder
    can find them
  * only expose "register_dencoders()" function from plugins.
  * load plugins in specified directory
* ceph.spec.in: package plugins
* debian: package plugins

Signed-off-by: Kefu Chai <kchai@redhat.com>