cfm: Allow accurate transmission intervals in extended mode.

cfm: Allow accurate transmission intervals in extended mode.

The standard CFM protocol only allows a handful of transmission
rates.  This is particularly problematic if you want to support a
transmission rate slower than 100 ms and faster than 1000 ms.

This patch allows arbitrary transmission rates (between 1 ms and
65535 ms).  It does this by commandeering parts of a reserved
"zero" field in the ccm message.  This breaks wire compatibility
with standard 802.1ag implementations, and thus is only supported
in extended mode.

Bug #7014.