pgsql-server: Clean up handling of inherited-table update queries, per

Log Message:
-----------
Clean up handling of inherited-table update queries, per bug report
from Sebastian Bck. The fix involves being more consistent about
when rangetable entries are copied or modified. Someday we really
need to fix this stuff to not scribble on its input data structures
in the first place...

Modified Files:
--------------
pgsql-server/src/backend/optimizer/path:
allpaths.c (r1.121 -> r1.122)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/optimizer/path/allpaths.c.diff?r1=1.121&r2=1.122)
pgsql-server/src/backend/optimizer/plan:
planner.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/optimizer/plan/planner.c.diff?r1=1.175&r2=1.176)
pgsql-server/src/backend/optimizer/prep:
prepunion.c (r1.116 -> r1.117)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/optimizer/prep/prepunion.c.diff?r1=1.116&r2=1.117)
pgsql-server/src/backend/optimizer/util:
clauses.c (r1.180 -> r1.181)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/optimizer/util/clauses.c.diff?r1=1.180&r2=1.181)
pgsql-server/src/include/optimizer:
prep.h (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/optimizer/prep.h.diff?r1=1.45&r2=1.46)