Make AtomPub discoverable

[rubys]

It would help tools like  Windows Live Writer if WP's AtomPub support were discoverable. The simple patch attached does just that. It also corrects a minor typo in Atom Feed autodiscovery. Tested on latest HEAD.

Looks good.

atomsvc link should be in a new function not in rsd_link and then added to wp_head action in default_filters.

update patch based on suggestion from Elias Torres

It’d be really helpful if, in the RSD, the blogId attribute pointed to the collection HREF--that way clients don’t have to prompt the user to pick their blog from a list.

In ticket #5175, which I've now marked as a dupl of this ticket, adding to xmlrpc.php?rsd seems sufficient, and any technical benefit of explicitly including it in wp_head is not immediately obvious to me.

Replying to foolswisdom: The <link rel="service"> is how everyone else in the Atom community is doing this right now (or were until this came up for WP), and it's where the next version of WLW will look. (I don't count Six Apart, their Atom implementations are based on an incredibly old draft and don't seem like they have been updated in years.)

 http://jcheng.wordpress.com/2007/10/16/how-wlw-speaks-atompub-part-1-autodiscovery/

Following discussion with rubys / josephscott it is thought that just adding to the RSD is enough

See [6161]

(In [6262]) Add Atom to the RSD. Fixes #5213 props rubys.

Replying to rubys:

corrects a minor typo in Atom Feed autodiscovery.

Fixed in [6261].