create a define to represent the 'invalid' generation id we used in two

create a define to represent the 'invalid' generation id we used in two
places.

create a new helper function to generate new generation id values that
know about the invalid id and avoids generating it.

update the ctdb status tool to know about the invalid generation id and
print the string INVALID instead

(This used to be ctdb commit 4fbcd189543cb8a92227fdcd3d158472e558ccda)