- Remove "first-installed-win" logic for automatically setting

${PYTHON_DEFAULT_VERSION}, this generates conflicting packages.

- Create symbolic links as PEP 394 [1] suggests. ${PYTHON_DEFAULT_VERSION}
  will create python and python${MAJOR_VERSION} links.  In current default,
  lang/python27 will create: python -> python2 -> python2.7

- Introduce ${PYTHON3_DEFAULT_VERSION}, which will handle bin/python3 link.
  At this point, lang/python33 will create python3 -> python3.3

- Minor cleanups
  * Trim Makefile headers
  * Remove ${OSVERSION} detection for xz, whihc is done by USE_XZ

[1] http://www.python.org/dev/peps/pep-0394/

lang/python27/pkg-plist

@@ -1,21 +1,27 @@
 libdata/pkgconfig/python-2.7.pc
 %%IF_DEFAULT%%bin/2to3
+%%IF_DEFAULT%%bin/2to3-%%PYMAJOR%%
 bin/2to3-%%PYVER%%
 %%IF_DEFAULT%%bin/idle
+%%IF_DEFAULT%%bin/idle%%PYMAJOR%%
 bin/idle%%PYVER%%
 %%IF_DEFAULT%%bin/pydoc
+%%IF_DEFAULT%%bin/pydoc%%PYMAJOR%%
 bin/pydoc%%PYVER%%
 %%IF_DEFAULT%%bin/python
-bin/python%%PYMAJOR%%
+%%IF_DEFAULT%%bin/python%%PYMAJOR%%
 bin/python%%PYVER%%
 %%IF_DEFAULT%%bin/python-config
-bin/python%%PYMAJOR%%-config
+%%IF_DEFAULT%%bin/python%%PYMAJOR%%-config
 bin/python%%PYVER%%-config
 %%IF_DEFAULT%%bin/python-shared
+%%IF_DEFAULT%%bin/python-shared%%PYMAJOR%%
 bin/python-shared%%PYVER%%
 %%IF_DEFAULT%%bin/python-shared-config
+%%IF_DEFAULT%%bin/python-shared%%PYMAJOR%%-config
 bin/python-shared%%PYVER%%-config
 %%IF_DEFAULT%%bin/smtpd.py
+%%IF_DEFAULT%%bin/smtpd%%PYMAJOR%%.py
 bin/smtpd%%PYVER%%.py
 %%PYTHON_INCLUDEDIR%%/Python-ast.h
 %%PYTHON_INCLUDEDIR%%/Python.h