Fix hard-coded relkind constants in pg_dump.c.

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.  There were only
a few such violations, and all relatively new AFAICT.

Existing style is mostly to inject relkind values into constructed
query strings using %c.  I did not bother to touch places that did it
like that, but really a better technique is to stringify the RELKIND
macro, at least in places where you'd want single quotes around the
code character.  That avoids any runtime effort and keeps the RELKIND
symbol close to where it's used.

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