Switch post type and tax capabilities to a single 'cap' property

[nacin]

Like what we're doing in #12968 and #13357 with labels.

Thus:

$pt->cap->edit_post
$pt->cap->edit_posts
$pt->cap->edit_others_posts

Etc.

By basing them off what they are called for posts, we reduce confusion significantly (it's also the default) and we prevent errors like [13955] which caused quite a headache, and MarkJaquith and I spent maybe an hour tracking that down.

Patches welcome, otherwise I'll get to it.

[nacin]

(In [14584]) Use correct cap check. see #13358.

[nacin]

(In [14585]) Move post type object capabilities to a 'cap' object. Allow them to be initialized via the 'capabilities' key (an array) when registering support for the post type. Caps are now referred to by the name of the cap as if it was a post, i.e. ->cap->edit_post. see #13358.

[nacin]

(In [14587]) Fix typos. see #13358.

[nacin]

(In [14588]) More typos. see #13358.

[nacin]

(In [14589]) Unset capabilities property when we're done. see #13358.

[nacin]

If someone wants to take this up for taxonomies, please do.

[nacin]

(In [14593]) Move taxonomy object properties for capabilities into a cap object. Capabilities can be specified via capabilities? (an array keyed by the generic cap name) for register_taxonomy. fixes #13358.