Correct constness of a few variables.

This allows the compiler / linker to mark affected pages as read-only.

There's other cases, but they're a bit more invasive, and should go
through some review. These are easy.

They were found with
objdump -j .data -t src/backend/postgres|awk '{print $4, $5, $6}'|sort -r|less

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

src/backend/storage/lmgr/generate-lwlocknames.pl

@@ -23,7 +23,7 @@ print $h $autogen;
 print $h "/* there is deliberately not an #ifndef LWLOCKNAMES_H here */\n\n";
 print $c $autogen, "\n";
 
-print $c "char *MainLWLockNames[] = {";
+print $c "const char *const MainLWLockNames[] = {";
 
 while (<$lwlocknames>)
 {