pgsql: Refactor the handling of the various DropStmt variants so that

Log Message:
-----------
Refactor the handling of the various DropStmt variants so that when multiple
objects are specified, we drop them all in a single performMultipleDeletions
call.  This makes the RESTRICT/CASCADE checks more relaxed: it's not counted
as a cascade if one of the later objects has a dependency on an earlier one.
NOTICE messages about such cases go away, too.

In passing, fix the permissions check for DROP CONVERSION, which for some
reason was never made role-aware, and omitted the namespace-owner exemption
too.

Alex Hunsaker, with further fiddling by me.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        dependency.c (r1.75 -> r1.76)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/dependency.c?r1=1.75&r2=1.76)
        pg_conversion.c (r1.43 -> r1.44)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_conversion.c?r1=1.43&r2=1.44)
    pgsql/src/backend/commands:
        conversioncmds.c (r1.33 -> r1.34)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c?r1=1.33&r2=1.34)
        indexcmds.c (r1.176 -> r1.177)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/indexcmds.c?r1=1.176&r2=1.177)
        schemacmds.c (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/schemacmds.c?r1=1.49&r2=1.50)
        tablecmds.c (r1.255 -> r1.256)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.255&r2=1.256)
        tsearchcmds.c (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tsearchcmds.c?r1=1.11&r2=1.12)
        typecmds.c (r1.118 -> r1.119)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c?r1=1.118&r2=1.119)
        view.c (r1.104 -> r1.105)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/view.c?r1=1.104&r2=1.105)
    pgsql/src/backend/tcop:
        utility.c (r1.292 -> r1.293)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.292&r2=1.293)
    pgsql/src/include/catalog:
        pg_conversion_fn.h (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_conversion_fn.h?r1=1.1&r2=1.2)
    pgsql/src/include/commands:
        conversioncmds.h (r1.16 -> r1.17)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/conversioncmds.h?r1=1.16&r2=1.17)
        defrem.h (r1.88 -> r1.89)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/defrem.h?r1=1.88&r2=1.89)
        schemacmds.h (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/schemacmds.h?r1=1.18&r2=1.19)
        tablecmds.h (r1.38 -> r1.39)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/tablecmds.h?r1=1.38&r2=1.39)
        typecmds.h (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/typecmds.h?r1=1.23&r2=1.24)
        view.h (r1.26 -> r1.27)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/view.h?r1=1.26&r2=1.27)
    pgsql/src/test/regress/expected:
        foreign_key.out (r1.47 -> r1.48)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/foreign_key.out?r1=1.47&r2=1.48)
        truncate.out (r1.16 -> r1.17)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/truncate.out?r1=1.16&r2=1.17)
    pgsql/src/test/regress/sql:
        foreign_key.sql (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/foreign_key.sql?r1=1.21&r2=1.22)