Improve global variable setting in `setup_postdata()`.

`setup_postdata()` is responsible for setting a number of global variables
that are used for post pagination (`$pages`, `$page`, `$nextpage`) and the
generation of post excerpts (`$more`). These variables should be sensitive to
the currently running instance of `WP_Query` - rather than the main query -
so that these features work properly inside of secondary `WP_Query` loops.

This changeset moves the logic of `setup_postdata()` into a method on `WP_Query`,
and converts `setup_postdata()` to a wrapper.

Props boonebgorges, wonderboymusic.
See #25349.
Fixes #9256, #20904.

git-svn-id: https://develop.svn.wordpress.org/trunk@30085 602fd350-edb4-49c9-b593-d223f7449a82