Sync our DTrace infrastructure with c.h's definition of type bool.

Since commit d26a810eb, we've defined bool as being either _Bool from
<stdbool.h>, or "unsigned char"; but that commit overlooked the fact
that probes.d has "#define bool char".  For consistency, make it say
"unsigned char" instead.  This should be strictly a cosmetic change,
but it seems best to be in sync.

Formally, in the now-normal case where we're using <stdbool.h>, it'd
be better to write "#define bool _Bool".  However, then we'd need
some build infrastructure to inject that configuration choice into
probes.d, and it doesn't seem worth the trouble.  We only use
<stdbool.h> if sizeof(_Bool) is 1, so having DTrace think that
bool parameters are "unsigned char" should be close enough.

Back-patch to v12 where d26a810eb came in.

Discussion: https://postgr.es/m/CAA4eK1LmaKO7Du9M9Lo=kxGU8sB6aL8fa3sF6z6d5yYYVe3BuQ@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -4638,7 +4638,7 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
     </row>
     <row>
      <entry><type>bool</type></entry>
-     <entry><type>char</type></entry>
+     <entry><type>unsigned char</type></entry>
     </row>
 
    </tbody>