Make nntplib aware of ~/.netrc credentials; now they get used if they are

Make nntplib aware of ~/.netrc credentials; now they get used if they are
present and the caller has not specified a name/password pair.  This change
makes it less likely that a lazy coder will expose sensitive information in a
word-readable script.

Also, make the test a bit smarter.  If NNTPSERVER is defined in the environment
it will go talk to that server rather than look for a possibly nonexistent
local one named 'news'.  Maybe the osession initializer ought to look at
NNTPSERVER rather than requiring a host arg?  Must look around and see how
universal this convention is first.