pgsql: DROP IF EXISTS for columns and constraints.

Log Message:
-----------
DROP IF EXISTS for columns and constraints. Andres Freund.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
alter_table.sgml (r1.106 -> r1.107)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_table.sgml?r1=1.106&r2=1.107)
pgsql/src/backend/commands:
tablecmds.c (r1.290 -> r1.291)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.290&r2=1.291)
pgsql/src/backend/nodes:
copyfuncs.c (r1.433 -> r1.434)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.433&r2=1.434)
equalfuncs.c (r1.356 -> r1.357)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.356&r2=1.357)
pgsql/src/backend/parser:
gram.y (r2.670 -> r2.671)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.670&r2=2.671)
pgsql/src/include/nodes:
parsenodes.h (r1.396 -> r1.397)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.396&r2=1.397)
pgsql/src/test/regress/expected:
alter_table.out (r1.115 -> r1.116)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out?r1=1.115&r2=1.116)
pgsql/src/test/regress/sql:
alter_table.sql (r1.61 -> r1.62)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql?r1=1.61&r2=1.62)