pgsql: Fix ALTER SEQUENCE so that it does not affect the value of

Log Message:
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Fix ALTER SEQUENCE so that it does not affect the value of currval() for
the sequence. Also, make setval() with is_called = false not affect the
currval state, either. Per report from Kris Jurka that an implicit
ALTER SEQUENCE OWNED BY unexpectedly caused currval() to become valid.
Since this isn't 100% backwards compatible, it will go into HEAD only;
I'll put a more limited patch into 8.2.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.404 -> r1.405)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.404&r2=1.405)
pgsql/doc/src/sgml/ref:
alter_sequence.sgml (r1.18 -> r1.19)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_sequence.sgml?r1=1.18&r2=1.19)
pgsql/src/backend/commands:
sequence.c (r1.146 -> r1.147)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/sequence.c?r1=1.146&r2=1.147)