Provide real selectivity estimators for inet/cidr operators.

This patch fills in the formerly-stub networksel() and networkjoinsel()
estimation functions.  Those are used for << <<= >> >>= and && operators
on inet/cidr types.  The estimation is not perfect, certainly, because
we rely on the existing statistics collected for the inet btree operators.
But it's a long way better than nothing, and it's not clear that asking
ANALYZE to collect separate stats for these operators would be a win.

Emre Hasegeli, with reviews from Dilip Kumar and Heikki Linnakangas,
and some further hacking by me

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