pgsql: Fix saveHistory() to not emit bogus complaint during psql exit

Log Message:
-----------
Fix saveHistory() to not emit bogus complaint during psql exit when using
Darwin's libedit; per my proposal last August. Also, suppress cast-away-
const warnings.

Modified Files:
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pgsql/src/bin/psql:
input.c (r1.55 -> r1.56)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/input.c.diff?r1=1.55&r2=1.56)