pgsql: Utilize the visibility map in autovacuum, too.

Log Message:
-----------
Utilize the visibility map in autovacuum, too. There was an oversight in
the visibility map patch that because autovacuum always sets
VacuumStmt->freeze_min_age, visibility map was never used for autovacuum,
only for manually launched vacuums. This patch introduces a new scan_all
field to VacuumStmt, indicating explicitly whether the visibility map
should be used, or the whole relation should be scanned, to advance
relfrozenxid. Anti-wraparound vacuums still need to scan all pages.

Modified Files:
--------------
    pgsql/src/backend/commands:
        vacuumlazy.c (r1.112 -> r1.113)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.112&r2=1.113)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.413 -> r1.414)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.413&r2=1.414)
        equalfuncs.c (r1.338 -> r1.339)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.338&r2=1.339)
    pgsql/src/backend/parser:
        gram.y (r2.641 -> r2.642)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.641&r2=2.642)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.87 -> r1.88)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.87&r2=1.88)
    pgsql/src/include/nodes:
        parsenodes.h (r1.379 -> r1.380)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.379&r2=1.380)