Introduce `get_previous_post_link()`, `get_next_post_link()`, and `get_adjacent_post_link()`. Allows developers to retrieve the value without `echo`ing.

Props yoavf, markjaquith, SergeyBiryukov.
Fixes #17302.


Built from https://develop.svn.wordpress.org/trunk@25327


git-svn-id: http://core.svn.wordpress.org/trunk@25289 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add filter `flush_rewrite_rules_hard` to give devs chance to prevent writing to file when hard-flushing rewrite rules.

Props jeremyfelt, nacin.
Fixes #23491.


Built from https://develop.svn.wordpress.org/trunk@25328


git-svn-id: http://core.svn.wordpress.org/trunk@25290 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add an additional available param (the `WP_User` instance) to the `user_has_cap` filter.

Props Viper007Bond.
Fixes #24490.


Built from https://develop.svn.wordpress.org/trunk@25329


git-svn-id: http://core.svn.wordpress.org/trunk@25291 1a063a9b-81f0-0310-95a4-ce76da25c4cd