Thanks Michael Jung, Charl Botha and Florian Apolloner for review
and help on the patch.

Also made some import cleanups while I was there.

Refs #10399.

Thanks ratsonale for spotting this mistake.

This was used at the time the 'database' option did not default to
DEFAULT_DB_ALIAS in make_option.

Thanks edd at slipszenko.net for the report.

Thanks Simon Charette for the review.

Thanks tomas_00 for the report.

Thanks Anssi Kääriäinen and Karen Tracey for their inputs.

Also added a little improvement to sql/query.py to get rid of
non-necessary IS NOT NULL check.

Thanks to Bruno Renie for reporting.

Fixed two test failures when tmp dirs are below a symlink.

This affects OS X where /var is a symlink to /private/var.

Fixed a typo in Python 3 porting guide.

Thanks Randy Salvo for the report.

* StandardError doesn't exist any more under Python 3.
* Python 2.6 still allows `raise "foo"`.

Squashed commit of the following:

commit 2181d833ed1a2e422494738dcef311164c4e097e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 27 14:28:39 2013 +0100

    Fixed #15901 -- Wrapped all PEP-249 exceptions.

commit 5476a5d93c19aa2f928c497d39ce6e33f52694e2
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 17:26:52 2013 +0100

    Added PEP 3134 exception chaining.

    Thanks Jacob Kaplan-Moss for the suggestion.

commit 9365fad0a650328002fb424457d675a273c95802
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 17:13:49 2013 +0100

    Improved API for wrapping database errors.

    Thanks Alex Gaynor for the proposal.

commit 1b463b765f2826f73a8d9266795cd5da4f8d5e9e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 15:00:39 2013 +0100

    Removed redundant exception wrapping.

    This is now taken care of by the cursor wrapper.

commit 524bc7345a724bf526bdd2dd1bcf5ede67d6bb5c
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 14:55:10 2013 +0100

    Wrapped database exceptions in the base backend.

    This covers the most common PEP-249 APIs:
    - Connection APIs: close(), commit(), rollback(), cursor()
    - Cursor APIs: callproc(), close(), execute(), executemany(),
      fetchone(), fetchmany(), fetchall(), nextset().

    Fixed #19920.

commit a66746bb5f0839f35543222787fce3b6a0d0a3ea
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 14:53:34 2013 +0100

    Added a wrap_database_exception context manager and decorator.

    It re-throws backend-specific exceptions using Django's common wrappers.

There were a couple of errors in ._dirty flag handling:
  * It started as None, but was never reset to None.
  * The _dirty flag was sometimes used to indicate if the connection
    was inside transaction management, but this was not done
    consistently. This also meant the flag had three separate values.
  * The None value had a special meaning, causing for example inability
    to commit() on new connection unless enter/leave tx management was
    done.
  * The _dirty was tracking "connection in transaction" state, but only
    in managed transactions.
  * Some tests never reset the transaction state of the used connection.
  * And some additional less important changes.

This commit has some potential for regressions, but as the above list
shows, the current situation isn't perfect either.

Thanks Preston Holmes for the text.

This allows us to iremove an inter-command import.

Fixes #19584.

This implies stop storing file path command line arguments in envvars as
a security measure to start relying on with Popen's shell=False instead,
and addition of an 'utils' module.

Thanks kmichel_wgs for the report.

Forward port of ab6ff2e0 from stable/1.5