Fix: allow -r to set from after -Sfrom=, II. (Michael Dressel)..

Fix: allow -r to set from after -Sfrom=, II. (Michael Dressel)..

This is the full commit including test, the essential fix had been
applied to [master] immediately.

Michael Dressel (dressel at mail dot desy dot de) reported a bug
introduced in [94bafbc6b4] (Temporarily freeze variables set via
-S.., 2017-09-03): if *from* would have been set via -S somehow,
-r would have not been allowed to overwrite that freeze.
(Funnily Desy seems to be great science and operates super
lasers which also heat water to 100000 degree Celsius in 75
microseconds, and even more.)

Despite this the situation is pretty frustrating and TODO contains
since v14.9.8 i think this

- -r should be the Sender:, which should automatically propagate to
  From: if possible and/or necessary.  It should be possible to suppress
  -r stuff from From: and Sender:, but fallback to special -r arg as
  appropriate.

Sigh.