Convert the Python framework bits to USES=python.

Please use USES=python instead of USE_PYTHON.

USE_PYTHON=yes becomes USES=python
USE_PYTHON=2.7+ becomes USES=python:2.7+
USE_PYTHON_BUILD=3.3 becomes USES=python:3.3,build
...

A new PYTHON_FEATURES variable was added, which enables certain features for a
port and replaces some knobs at the same time;

PYTHON_FEATURES=distutils replaces USE_PYDISTUTILS
PYTHON_FEATURES=autoplist replaces PYDISTUTILS_AUTOPLIST
PYTHON_FEATURES=py3kplist replaces PYTHON_PY3K_PLIST_HACK
PYTHON_FEATURES=noegginfo replaces PYDISTUTILS_NOEGGINFO
PYTHON_FEATURES=concurrent replaces PYTHON_CONCURRENT_INSTALL
PYTHON_FEATURES=pythonprefix replaces USE_PYTHON_PREFIX

Some knobs have been deprecated and are to be removed in the near future:

PYTHON_MASTER_SITES - use MASTER_SITE_PYTHON instead
PYTHON_PKGNAMESUFFIX - use PYTHON_PKGNAMEPREFIX instead
PYDISTUTILS_INSTALLNOSINGLE - deprecated without replacement

Some knobs have been removed completely:

PYTHON_MASTER_SITE_SUBDIR
PYTHON_DISTNAME
PYTHON_WRKSRC

Several variables specific to the Python framework are no longer passed to the
build environment to avoid polluting dependency builds.

PYTHON_VERSION is not passed to .MAKEFLAGS anymore
PYTHON_DEFAULT_VERSION,
PYTHON_DEFAULT_PORTVERSION and
PYTHONBASE are not passed to the make environment anymore

The conversion required a couple of ports to be updated to fit the changes and
new requirements. Those included "bsd.python.mk" directly or contained checks
in places, for which the USES framework would fail to provide correct values.

Python modules directly using the upstream Python package (such as py-tkinter
or py-sqlite3) were updated to avoid using the now unnecessary and remmoved
knobs from "bsd.python.mk".

Phabric: D399
exp-run: 167368 192357
PR: 167368 192357
Reviewed by: antoine, wg
Exp-run award: antoine
With hat: python@
Approved by: portmgr