Fix hard-coded relkind constants in assorted other files.

Fix hard-coded relkind constants in assorted other files.

Although it's reasonable to expect that most of these constants will
never change, that does not make it good programming style to hard-code
the value rather than using the RELKIND_FOO macros.

I think I've now gotten all the hard-coded references in C code.
Unfortunately there's no equally convenient way to parameterize
SQL files ...

Discussion: https://postgr.es/m/11145.1488931324@sss.pgh.pa.us