Online (front-end) membershp renewals don't set correct default membership type AND On-behalf of renewals don't update inherited memberships

To recreate:
1. Add new membership type "Company" with inherited Relationship Type = Employer of
2. Modify sample "Membership Signup and Renewal" online contribution page to allow "On behalf ..." and to use a Dummy processor (so u can easily do live membership / contributions. Add "Company" membership type to Membership Block (default is still "General").
3. Sign up for company membership with "on behalf" of "Test Organization" etc.
4. Verify the inherited membership for your contact record and note down membership end_date.
5. Return to civicrm/contribute/transact?reset=1&id=2 after visiting another page.

You'll see the first bug:
The page "knows" the you have an inherited "Company" membership which expires on xxx date. BUT it still selects the default membership type "General". This is wrong behavior for renewal, we should select the person's existing membership type by default. Pretty sure this is a regression bug.

6. Select the "Company" membership type and check "on behalf", select existing organization and fill in "Test Organization" in autocomplete.
7. Complete this contribution / renewal and go check your inherited membership again.

Second bug: Your inherited membership end date has not been incremented. (Although the direct membership of "Test Organization" is correctly incremented to reflect the renewal).