pg_resetwal prints new OldestXID in wrong circumstances

When the --oldest-transaction-id option is used, pg_resetwal doesn't
print the new value like it does for other similar options. For example,
if you use --next-oid and --oldest-transaction-id, only the new NextOID
value is printed:

> $ pg_resetwal --dry-run -D data --next-oid 10 --oldest-transaction-id 100
> Current pg_control values:
>
> ...
>
> Values to be changed:
>
> First log segment after reset: 000000010000000000000005
> NextOID: 10

Printing the new OldestXID value is incorrectly tied to whether the
--next-transaction-id option is given, so this prints it, even though
OldestXID is not being modified:

> $ pg_resetwal --dry-run -D data --next-oid 10 --next-transaction-id 100
> Current pg_control values:
>
> ...
>
>
> Values to be changed:
>
> First log segment after reset: 000000010000000000000005
> NextOID: 10
> NextXID: 100
> OldestXID: 744
> OldestXID's DB: 1

This seems to have been an oversight when the --oldest-transaction-id
option was added (commit 74cf7d46a91d). Before that, OldestXID was reset
when the --next-transaction-id option was given.

Fix attached. Barring objections, I will commit and backpatch this.

- Heikki