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.

do not contain null bytes.

NULL bytes it would cause the database all sorts of problems in the future
leading to very strange random failures and corrupt dbtables.bsdTableDb dbs.

keyword argument whenever possible to avoid bugs and confusion.  (dbtables.py
line 447 self.db.get using txn as a non-keyword was an actual bug due to this)

it closes itself.  When the stream is read in several calls to read(n),
it should behave in the same way if HTTPConnection knows where the end
of the stream is (through self.length).  Added a test case for this
behaviour.