logging minibuffer-message messages

[Drew Adams]

How about having a way to log minibuffer messages?

For example, option `message-log-max', which allows for disabling logging, could
have additional values:

* negative N means (abs N) max lines + log minibuf msgs too
* other non-nil value, besides t and N, means log minibuf msgs too

With minibuffer messages logged also, a user can consult *Messages* (e.g. using
`C-x C-b') if s?he misses an ephemeral message.