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)