Add alert to admin page about old PHP version.

Add an alert message to admin summary page when a PHP version < 5.4 was
detected as this has reached EOL and will not be supported with the new
SabreDAV library.
Some styleguide optimisation.
Add NULL_DATE example to doc/sql_conventions.bb.
Display a "0" when no plugins active in summary page.
Added some more numbers to summary admin page, expired accounts,
channels. Hope I interpreted them right. I have no idea how to get
blocked accounts.

doc/sql_conventions.bb

@@ -56,7 +56,11 @@ $r = q("SELECT * FROM mail WHERE uid=%d AND $sql_extra ORDER BY created DESC LIM
 
 [b]NULL dates[/b]
 [li]To be written
-[code]Example[/code][/li]
+[code]// Example
+$r = q("DELETE FROM mail WHERE expires != '%s' AND expires < %s ",
+	dbesc(NULL_DATE),
+	db_utcnow()
+);[/code][/li]
 
 [b]Storing binary data[/b]
 [li]To be written