I was annoyed, message-user-mail-address function uses the
user-mail-address variable only. It may be desireable in some cases to
have it derived from the From header (esp, in the
message-send-mail-with-sendmail function).

This patch seems to do it. But I'm really not sure, if it breaks
something.

BTW, maybe, we should add a check for Errors-To header prior to From?
Opinions?