add 'can_comment_on_post' hook so we can better deal with the complications of Diaspora policy

doc/hook/can_comment_on_post.bb

@@ -0,0 +1,13 @@
+[h3]can_comment_on_post[/h3]
+
+Called when deciding whether or not to display a comment box for a post.
+
+
+Hook data (array):
+	observer_hash => xchan_hash of current observer
+	item => posted item
+	allowed => 'unset'
+
+
+To over-ride the default behaviour, change allowed to true or false
+

doc/hooklist.bb

@@ -82,6 +82,9 @@ Hooks allow plugins/addons to "hook into" the code at many points and alter the
 [zrl=[baseurl]/help/hook/build_pagehead]build_pagehead[/zrl]
 	Called when creating the HTML page header
 
+[zrl=[baseurl]/help/hook/can_comment_on_post]can_comment_on_post[/zrl]
+	Called when deciding whether or not to present a comment box for a post
+
 [zrl=[baseurl]/help/hook/change_channel]change_channel[/zrl]
 	Called when logging in to a channel (either during login or afterward through the channel manager)
 

include/items.php

@@ -219,6 +219,11 @@ function can_comment_on_post($observer_xchan, $item) {
 
 //	logger('can_comment_on_post: comment_policy: ' . $item['comment_policy'], LOGGER_DEBUG);
 
+	$x = [ 'observer_hash' => $observer_xchan, 'item' => $item, 'allowed' => 'unset' ];
+	call_hooks('can_comment_on_post',$x);
+	if($x['allowed'] !== 'unset')
+		return $x['allowed'];
+
 	if(! $observer_xchan)
 		return false;