Change mentions of PuSH to WebSub

WebSub is probably finalised before we make a release anyway. Here is
the official spec: https://www.w3.org/TR/websub/

Mostly just comments that have been changed. Some references to PuSH <0.4
are left because they actually refer to PuSH 0.3 and that's not WebSub...

The only actual code change that might affect anything is FeedSub->isPuSH()
but the only official plugin using that call was FeedPoller anyway...

plugins/OStatus/scripts/resub-feed.php

@@ -25,7 +25,7 @@ $shortoptions = 'u';
 
 $helptext = <<<END_OF_HELP
 resub-feed.php [options] http://example.com/atom-feed-url
-Reinitialize the PuSH subscription for the given feed. This may help get
+Reinitialize the WebSub subscription for the given feed. This may help get
 things restarted if we and the hub have gotten our states out of sync.
 
 Options:

plugins/OStatus/scripts/testfeed.php

@@ -24,7 +24,7 @@ $longoptions = array('skip=', 'count=');
 
 $helptext = <<<END_OF_HELP
 testfeed.php [options] http://example.com/atom-feed-url
-Pull an Atom feed and run items in it as though they were live PuSH updates.
+Pull an Atom feed and run items in it as though they were live WebSub updates.
 Mainly intended for testing funky feed formats.
 
      --skip=N   Ignore the first N items in the feed.

plugins/OStatus/scripts/update-profile.php

@@ -24,7 +24,7 @@ $helptext = <<<END_OF_HELP
 update-profile.php [options] http://example.com/profile/url
 
 Rerun profile and feed info discovery for the given OStatus remote profile,
-and reinitialize its PuSH subscription for the given feed. This may help get
+and reinitialize its WebSub subscription for the given feed. This may help get
 things restarted if the hub or feed URLs have changed for the profile.