net/smtp: allow PLAIN auth when advertised

net/smtp: allow PLAIN auth when advertised

The smtp package originally allowed PLAIN whenever, but then
the TLS check was added for paranoia, but it's too paranoid:
it prevents using PLAIN auth even from localhost to localhost
when the server advertises PLAIN support.

This CL also permits the client to send PLAIN if the server
advertises it.

Fixes #5184

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8279043