Register the 'close' event listener with .once(), not .on().

It doesn't matter in the grand scheme of things because the listener
doesn't keep references to any heavy-weight objects but using .once()
for a oneshot listener is something of a best practice.

Remove the assert() that triggered when Cipher#final() or
Decipher#final() was called twice.

Fixes #4886.

This is a back-port of commit 17a8126 from the master branch.

Don't use hard-coded port numbers, use common.PORT instead.

Should fix the occasional Jenkins failure; the builds run in parallel.

Conflicts:
	doc/api/http.markdown
	test/simple/test-crypto.js