pgsql: Make DROP IF EXISTS more consistently not fail

Make DROP IF EXISTS more consistently not fail

Some cases were still reporting errors and aborting, instead of a NOTICE
that the object was being skipped.  This makes it more difficult to
cleanly handle pg_dump --clean, so change that to instead skip missing
objects properly.

Per bug #7873 reported by Dave Rolsky; apparently this affects a large
number of users.

Authors: Pavel Stehule and Dean Rasheed.  Some tweaks by Álvaro Herrera

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b152c6cd0de1827ba58756e24e18110cf902182a

Modified Files
--------------
src/backend/catalog/namespace.c              |   45 +++--
src/backend/catalog/objectaddress.c          |   46 +++--
src/backend/commands/dropcmds.c              |  258 +++++++++++++++++++++-----
src/backend/commands/functioncmds.c          |    4 +-
src/backend/commands/opclasscmds.c           |   26 ++-
src/backend/commands/tablecmds.c             |   26 ++-
src/backend/commands/typecmds.c              |    2 +-
src/backend/parser/parse_func.c              |   32 +---
src/backend/parser/parse_oper.c              |    4 +-
src/backend/parser/parse_type.c              |   88 +++++++--
src/backend/utils/adt/regproc.c              |    6 +-
src/include/catalog/namespace.h              |    3 +-
src/include/parser/parse_type.h              |    4 +-
src/pl/plpgsql/src/pl_comp.c                 |    2 +-
src/test/regress/expected/drop_if_exists.out |   79 +++++++-
src/test/regress/expected/event_trigger.out  |    2 +-
src/test/regress/sql/drop_if_exists.sql      |   41 ++++
17 files changed, 523 insertions(+), 145 deletions(-)