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Fix all PEP8 errors but E221 (multiple spaces before operator).

Instead of depending on config.py defining several magic variables,
move the default configuration in the config instance at the root of
the lib module, and make config.py override them as needed.

Continuous polling means that the process runs forever, checking
everything from time to time.
To get results, a listing of checks' state is printed when SIGUSR1
is caught. Single run (for config or debug) is accessible with the
-1 or --one command line switch.

Create a directory "lib/", put the checks there. Move the compat file
there too.
Also put the config in a separate file so that users may edit without
messing up the core. Would also ease intallation so that config
may land in etc/ for example.

Apparently, Popen.args is new and undocumented in Python 3.3, so don't
rely on its existence and set it ourselves.

Now the granularity is the check level, allowing to write checks
without hosts. Checks don't deal with hosts anymore.
Also the results of checks are now handled outside of the checking
path, but this may change when the infinite loop and time intervals
are going to be implemented.

IPy is only used to get the version of an IP (4 or 6), but the IPs come
from reasonably trustable sources and if incorrect would at worse make
the test fail -- which is valid behavior.

So, no need for a real library to distinguish them, do a naive check
based on the presence of version-specific delimiters.