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/replication/basebackup.c

@@ -190,7 +190,7 @@ static const char *excludeFiles[] =
 /*
  * List of files excluded from checksum validation.
  */
-static const char *noChecksumFiles[] = {
+static const char *const noChecksumFiles[] = {
 	"pg_control",
 	"pg_filenode.map",
 	"pg_internal.init",
@@ -1321,7 +1321,7 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 static bool
 is_checksummed_file(const char *fullpath, const char *filename)
 {
-	const char **f;
+	const char *const *f;
 
 	/* Check that the file is in a tablespace */
 	if (strncmp(fullpath, "./global/", 9) == 0 ||