pgsql: Extend "ALTER EXTENSION ADD object" to permit "DROP object" as w

Extend "ALTER EXTENSION ADD object" to permit "DROP object" as well.

Per discussion, this is something we should have sooner rather than later,
and it doesn't take much additional code to support it.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/01467d3e4f51fe304c0160c9895484ad696d1e87

Modified Files
--------------
doc/src/sgml/ref/alter_extension.sgml |   15 ++++-
src/backend/catalog/pg_depend.c       |   86 +++++++++++++++----------
src/backend/commands/extension.c      |   58 +++++++++++++----
src/backend/nodes/copyfuncs.c         |   11 ++--
src/backend/nodes/equalfuncs.c        |    7 +-
src/backend/parser/gram.y             |  113 +++++++++++++++++++-------------
src/backend/tcop/utility.c            |   10 ++--
src/bin/psql/tab-complete.c           |    7 ++-
src/include/catalog/dependency.h      |    3 +
src/include/commands/extension.h      |    2 +-
src/include/nodes/nodes.h             |    2 +-
src/include/nodes/parsenodes.h        |    5 +-
12 files changed, 207 insertions(+), 112 deletions(-)