Installer/Upgrader bugs and tweaks

[Denis-de-Bernardy]

Will post a big patch when I'm done testing the entire thing.

[Denis-de-Bernardy]

first batch attached, for plugins.

tested except for this part, which seemed very wrong:

-	$update_plugins = get_option('update_plugins');
+	$update_plugins = get_transient('update_plugins');

...

-	delete_option('update_plugins');
+	delete_transient('update_plugins');

a few comments on the patch:

• changes to wp-includes/update.php merely make the code conform with the comments
• the addition of the $res->external allows to remove the link to "Plugin page on WordPress.org" thingy, which is irrelevant when the plugin is hosted elsewhere
• $api->rating was missing a conditional statement

[ryan]

(In [11585]) Plugin install fixes. Props Denis-de-Bernardy. see #10192

[ryan]

(In [11586]) Plugin install fixes. Props Denis-de-Bernardy. see #10192 for 2.8.1

[Denis-de-Bernardy]

I'll open a new ticket if I find more.