pgsql: Marginal cleanup in arrangements for ensuring StrategyHintVacuum

Log Message:
-----------
Marginal cleanup in arrangements for ensuring StrategyHintVacuum is cleared
after an error during VACUUM. We have a PG_TRY block anyway around the only
call sites, so just reset it in the CATCH clause instead of having
AtEOXact_Buffers blindly do it during xact end. I think the old code was
actively wrong for the case of a failure during ANALYZE inside a
subtransaction --- the flag wouldn't get cleared until main transaction end.
Probably not worth back-patching though.

Modified Files:
--------------
pgsql/src/backend/commands:
vacuum.c (r1.338 -> r1.339)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.338&r2=1.339)
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.209 -> r1.210)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.209&r2=1.210)