pgsql: Modify vacuum() to accept a single relation OID instead of a list

Log Message:
-----------
Modify vacuum() to accept a single relation OID instead of a list (which we
always pass as a single element anyway.)  In passing, fix an outdated comment.

Modified Files:
--------------
    pgsql/src/backend/commands:
        vacuum.c (r1.374 -> r1.375)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.374&r2=1.375)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.78 -> r1.79)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.78&r2=1.79)
    pgsql/src/backend/tcop:
        utility.c (r1.291 -> r1.292)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.291&r2=1.292)
    pgsql/src/include/commands:
        vacuum.h (r1.76 -> r1.77)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/vacuum.h?r1=1.76&r2=1.77)