Neal: please backport!

When "MAIL" is received without args, an exception happens instead of
sending a 501 syntax error response.

while another thread uses it.

  configure and setup python on different platforms -- at least
  in theory.)
* Move the Python on Mac docs in that chapter.
* Add a new chapter about the command line invocation, by stargaming.

coefficient numbers, floats in the sign, and other details that
generated directly the wrong number in the best case, or triggered
misfunctionality in the alorithms.

Test cases added for these issues. Thanks Mark Dickinson.

See also https://bugs.launchpad.net/bugs/72505.

Ported from release25-maint branch.