bpo-29778: Ensure python3.dll is loaded from correct locations when Python is embedded.

(cherry picked from commit a6296d34a478b4f697ea9db798146195075d496c)

* bpo-30876: Relative import from unloaded package now reimports the package
instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.
(cherry picked from commit 8a9cd20edca7d01b68292036029ae3735ce65edd)

* bpo-18018: Import raises ImportError instead of SystemError if a relative
import is attempted without a known parent package.

* bpo-26367: importlib.__init__() raises ImportError like
builtins.__import__() when ``level`` is specified but without an accompanying
package specified.

Based on patch by Victor Stinner.

Add private C API function _PyUnicode_AsUnicode() which is similar to
PyUnicode_AsUnicode(), but checks for null characters..
(cherry picked from commit f7eae0adfcd4c50034281b2c69f461b43b68db84)

* bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to lock

This is especially important if PyThread_acquire_lock() is called reentrantly
(for example from a signal handler).

* Update 2017-06-26-14-29-50.bpo-30765.Q5iBmf.rst

* Avoid core logic when taking the mutex failed.
(cherry picked from commit f84ac420c2af98339678744953869cad3c253281)

In rare circumstances PyImport_Import() could return NULL without raising
an error.
(cherry picked from commit 145541c)

(cherry picked from commit 48fb766)