fix crashes when deallocating instances of cimported types after module cleanup, patch by Lisandro Dalcin

cython_inline: Stop passing unused `ctx` parameter to `cythonize`.

Update docs/src/userguide/external_C_code.rst

improve detection of the extension suffix for cython_inline

reliance on imp.get_suffixes()[0] broke cython_inline on Debian systems
with support of multiarch where the first entry corresponded to the multiarch
suffix while distutil were using non-multiarch version while actually building
the extension

Reference: http://mail.python.org/pipermail//cython-devel/2012-July/002933.html

Now it would cache the suffix used by distutils and reuse it within the
session