knet: Fix MTU sizes & allow transport config in corosync.conf

Corosync layers don't need to know the knet MTU size - this way
corosync fragments buffers only when they get larger than the
KNET buffer size (64K) and knet fragments below that based on
the actual MTU and transport considerations.

It is also now possible to configure knet to use UDP or SCTP
transports in corosync.conf. This is currently done per-link
so if you have more than 1 link you need several interface{}
stanzas inside totem{} to make it use other than the default
of UDP. if it's useful I might add the option of a global
default.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>