Expose quote_literal_cstr() from core.

Robert Haas <rhaas@postgresql.org>

Commit: 4343c0e546b216ab38a3397a4f0f7476d557b352
Author: Robert Haas <rhaas@postgresql.org>
Date: 2010-11-20T15:04:48Z
Releases: 9.1.0
Expose quote_literal_cstr() from core.

This eliminates the need for inefficient implementions of this
functionality in both contrib/dblink and contrib/tablefunc, so remove
them.  The upcoming patch implementing an in-core format() function
will also require this functionality.

In passing, add some regression tests.

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