totem.h: use symbolic array dimensions in public struct members

• include/corosync/totem/totem.h:

(TOTEM_PRIVATE_KEY_LEN, TOTEM_RRP_MODE_BYTES): Define.
(struct totem_config): Use the new names, rather than literals.

git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@2037 fd59a12c-fef9-0310-b244-a6a79926bd2f