enhancement for after_plugin_row hook

[strider72]

In 2.7 we added a new hook: after_plugin_row_$plugin_file

after_plugin_row fires after every single plugin row.

after_plugin_row_$plugin_file only fires after the particular plugin that matches $plugin_file.

WordPress Core still uses the older of the two, which is inefficient, and less flexible.

This patch change core so it uses the "_$plugin_file" version of the hooks when doing standard plugin update notices. This has two advantages:

1) More efficient. The hook only actually fires if a plugin has an update, instead of firing once for every single plugin.

2) It's now possible for plugins to manipulate the notice for a particular plugin (for example, a plugin might have reason to suppress or replace its own update notice). Previously only *all* notices can be turned off of changed -- not on a plugin by plugin basis.

It's a straightforward change. Somebody please commit this -- I could really use it for a "3rd-party update checker" I'm trying to put together.

[westi]

(In [11775]) Move plugin update notice output to the plugin specific hook so as to make them easier to filter and manage from plugins that want to do there own thing. Fixes #10464 props strider72.

[azaozz]

(In [11785]) Fix warnings in wp_plugin_update_rows(), see #10464