Change pg_dump to produce CREATE INDEX commands by using the backend's

Change pg_dump to produce CREATE INDEX commands by using the backend's
pg_get_indexdef() function, rather than reaching into the system catalogs
for itself.  This eliminates a fair amount of redundant code.  Also,
since I just changed pg_get_indexdef() to suppress display of default
index opclasses, this will mean that 7.2 and later dumps will not mention
opclasses unless they are non-default opclasses.  Should make life easier
for future index opclass reorganizations.