Fix #14379: AJAX requests on unload
(cherry picked from commit f9d41ac641dcb5a93ba8a9027476b160d8f41111)

Conflicts:

	src/ajax/xhr.js
Ref f9d41ac6: Opera-compatible form target
(cherry picked from commit d5b7139abce9dd207773531d6dcb3493c6ac0c7c)
Ref 705216dc: Synchronize ajax/xhr.js with master. Close gh-1418.