compat: consolidate definitions of osx and freebsd

compat: consolidate definitions of osx and freebsd

on osx, ENODATA = 96, so we need to fix it. also define
CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE for osx, ceph_time.h
defines this also, but i don't want to include compat.h in ceph_time.h
at this moment.

and silence the warning of
 #warning ENODATA already defined to a value different from 87 (ENOATRR), refining to fix
because it is fired everywhere on osx when "compat.h" is included.

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