Update URLs and mailing list addresses

Now that we moved our infrastructure away from SourceForge, update the
URLs and mailing list addresses accordingly.
README: Clarify license notice

Note that we permit redistribution and modification under the terms of
future GPL versions, and state the exclusion of warranty.  This is in
accordance with the license notices at the top of our source files.
README: Change GitHub URL

Link to our main project home page on GitHub instead of referencing only
the "nagios-plugins.git" repository.  (And put the URL on its own line.)