DRTVWR-474: Make login coroutine sync with updater process on failure.

Specifically, introduce an LLEventMailDrop("LoginSync"). When the updater
detects that an update is required, it will post to that rendezvous point.

When login.cgi responds with login failure, make the login coroutine wait (a
few seconds) for that ping from the updater.

If we receive that ping and if it contains a "reply" key, make the fail.login
listener respond to the updater with an indication of whether to proceed with
update.

If both login.cgi and the updater concur that an update is required, produce a
new confirmation message for the user and then (once user responds) tell the
updater to proceed. Otherwise, produce the usual login-failure message and
tell the updater never mind.

Introduce LLCoro::OverrideConsuming to provide temporary save/restore of the
set_consuming() / get_consuming() flag. It's a good idea to set the consuming
flag when retrieving data from an LLEventMailDrop.