unicode paths with embedded null character on Windows instead of silently
truncate them.

Based on patch by Gabriel Genellina.

Patch written by R. David Murray.

* trace_vdelete() with wrong mode no longer break tracing
* trace_vinfo() now always returns a list of pairs of strings

* Set but unused variable in Parser/pgen.c in non-debug builds. Patch by
  Christian Heimes.
* Unused static function in Modules/readline.c. Patch by Georg Brandl.
* main_window unused in Modules/tkappinit.c. Patch by Gregory P. Smith.
* Dead assignment in Modules/_ctypes/cfield.c. Extracted from patch by Brett
  Cannon.
* Expression result unused in PyObject_INIT macro expansions. Based on
  patches by Christian Heimes.
* Load expat_config.h and therefore pyconfig.h before C stdlib headers are
  loaded. This silences pre-processor warnings including '_POSIX_C_SOURCE
  redefined'. Extracted from patch by Christian Heimes.

represented as a compination of known flags.

This is how paste work on Windows, Mac, modern Linux apps, and ttk widgets.
The exception was X11 tk widgets.  Original patch by Serhiy Storchake.

  unused and outdated icons.

This matches the usage by BufferedReader, BufferedWriter, etc. Also document
and test that the write() methods should only access their argument before
they return.

Replace os.popen() with subprocess.Popen.

If the "gcc", "cc" or "objdump" command is not available, the code was
supposed to raise an OSError exception. But there was a bug in the code. The
shell code returns the exit code 10 if the required command is missing, and the
code tries to check for the status 10. The problem is that os.popen() doesn't
return the exit code directly, but a status which should be processed by
os.WIFEXITED() and os.WEXITSTATUS(). In practice, the exception was never
raised. The OSError exception was not documented and ctypes.util.find_library()
is expected to return None if the library is not found.

Based on patch by Victor Stinner.