function lroundups_migrate_options() appears to always overwrite existing options with old options

[benlk]

https://github.com/INN/link-roundups/blob/1477b0fddfcaa3ea2e4ecbb8c410937163b5fc72/inc/updates/functions.php#L22-L43

This was noticed when updating Link Roundups using the update button on all production sites. Sites with custom values for the following items were reset:

• single roundup name
• multiple roundups name
• roundup post-type slug
• custom markup for displaying saved links

The affected sites are listed in an email to Adam, and there's only 2, but that's enough that we're going to need to fix this eventually.

• [x] only update the new setting from the old setting if the new setting is not set, which involves changing this bit:

    // check for old options and replace the old prefixes with lroundups_
    foreach ( $old_options as $old_option ) {
        $new_option = str_replace(
            array('argo_link_roundups_', 'link_roundups_'), 'lroundups_', $old_option);
        $old_value = get_option($old_option);
        $result = update_option($new_option, $old_value);
        if ($result)
            delete_option($old_option);
    }

[aschweigert]

not super urgent, but we'll definitely want to make sure we address this before we push the next update