Ensure that user-mail-address always has a value

Ensure that user-mail-address always has a value

* lisp/startup.el (user-mail-address): Initialize in the normal way.
(command-line): Reset user-mail-address if needed using
standard custom machinery.
* lisp/mail/feedmail.el (feedmail-fiddle-from):
* lisp/mail/rmail.el (rmail-unknown-mail-followup-to):
* lisp/mail/rmailsum.el (rmail-header-summary):
Simplify now that user-mail-address is always set.
; * doc/lispref/os.texi (System Environment): Remove fixme comment.