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