Contains and is contained by operators of inet datatypes

Attached patch adds <@, @>, <<@, and @>> operator symbols for inet
datatype to replace <<=, >>=, <<, and >>. <@ and @> symbols are used
for containment for all datatypes except inet, particularly on the
geometric types, arrays; cube, hstore, intaray, ltree extensions.

<@ and @> symbols are standardised as on version 8.2 by Tom Lane at
2006 [1]. The previous symbols are left in-place but deprecated. The
patch does exactly the same for inet datatypes.

The << and >> are standard symbols for strictly left of and strictly
right of operators. Those operators would also make sense for inet
datatypes. If we make this change now; we can remove the symbols, and
reuse them for new operators in distant future.

The patch removes the recently committed SP-GiST index support for the
existing operator symbols to give move reason to the users to use the
new symbols. This change will also indirectly deprecate the
undocumented non-transparent btree index support that works sometimes
for some of the existing operators [2].

The patch includes changes on the regression tests and the
documentation. I will add it to 2016-11 Commitfest.

[1] https://www.postgresql.org/message-id/14277.1157304939@sss.pgh.pa.us
[2] https://www.postgresql.org/message-id/389.1042992616@sss.pgh.pa.us