Close #6203: Document that Python 3 sets LC_CTYPE at startup to the user's preferred locale encoding

before their mutexes.

open() and io.TextIOWrapper are now calling locale.getpreferredencoding(False)
instead of locale.getpreferredencoding() in text mode if the encoding is not
specified. Don't change temporary the locale encoding using locale.setlocale(),
use the current locale encoding instead of the user preferred encoding.

Explain also in open() documentation that locale.getpreferredencoding(False) is
called if the encoding is not specified.

- remove duplicate netmask/hostmask code
- make two ifs more pythonic
- remove packed property for networks
- some minor pep8 stuff
- Test coverage is now at 97%, the rest are mostly unreachable safeguards.

The fileobj argument was added during the 3.3 development cycle, so this change
does not break backward compatibility with 3.2.

This clears up an error in detected by refleak mode that showed up when
test_smtplib was run after test_smtpd in the same refleak run.

_BaseNetwork contained (faulty) methods for creating string representations.
I've fixed them and put them to use by eliminating identical overrides.

ipaddress._BaseV4.is_unspecified() compared IP addresses using "in" which
fails.

break someone's over specified test that depends on the exact error message.

when the path existed and had the S_ISGID mode bit set when it was
not explicitly asked for.  This is no longer an exception as mkdir
cannot control if the OS sets that bit for it or not.

when the path existed and had the S_ISGID mode bit set when it was not
explicitly asked for.  This is no longer an exception as mkdir cannot control
if the OS sets that bit for it or not.

Issue 14673: Add sys.implementation