ofproto: Add separate functions for checking bfd/cfm status change.

ofproto: Add separate functions for checking bfd/cfm status change.

Currently, ofproto_port_get_bfd/cfm_status() is used to check the
bfd/cfm status change and query the status change.  Users decide
what to do with the filled status struct based on the return value
of the funciton.  Such design is confusing and makes the caller
code hard to read.

This commit breaks the function into a status change check function
and a status query function, so that they become easier to read and
use.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>