libceph: handle new osdmap down/state change encoding

Old incrementals encode a 0 value (nearly always) when an osd goes down.
Change that to allow any state bit(s) to be flipped.  Special case 0 to
mean flip the CEPH_OSD_UP bit to mimic the old behavior.

Signed-off-by: Sage Weil <sage@newdream.net>