Changed to sort on add from sort on process for efficiency.  Some
events (e.g. AUTH_ACL_CHECK) could be trigged many times in a single
page request.

Allows a sequence integer to be supplied when registering to handle
an event.  When processing an event, handlers (hooks) will be
executed in ascending order of their sequence number.  If two or
more handlers have the same sequence number their order of
execution is undefined.

A handler wanting to be first, should use -PHP_MAX_INT.
A handler wanting to be last can use PHP_MAX_INT.
There may be conflicts if two or more plugins use these values in
the expectation that they will guarantee being first or last.

fix fs#2851
related to http://bugs.jqueryui.com/ticket/8761

This should make it possible to see if mod_security is involved,
at least on servers running mod_php.

if a needed CONNECT tunnel fails

Let's stop confusing people with what this button does

Needs a check for null now.

In most cases this change will have no effect, but noes the response will use the filename that was originally requested. The downloaded filename can be modified to something different as well. E.g. the siteexport plugin will make use of it.