Makefile: fix /sbin vs /usr/sbin behavior

Instead of telling configure to put things in /sbin, explicitly put the
two important items (mkcephfs and mount.fuse.ceph) in /sbin via an
automake rule.  This unbreaks FreeBSD 9.1 and probably others.

Based on patches originally from Alan Somers <asomers@gmail.com>, modified
for the current Makefile structure and applied to the specfile too.

Fixes: #6456
Signed-off-by: Sage Weil <sage@inktank.com>