Rename internal Python functions to start with an underscore

Rename internal Python functions to start with an underscore

I could not tell if GdbSetPythonDirectory is internal or not because
I could not find any references to it, so I left it as-is.

Tested by running the testsuite on gdb.python/*.exp; everything still
passes.

2019-08-15  Christian Biesinger  <cbiesinger@google.com>

	* python/lib/gdb/__init__.py (GdbOutputFile): Rename to have a
	leading underscore.
	(GdbOutputErrorFile): Likewise.
	(global scope): Adjust constructor calls to GdbOutput{,Error}File
	accordingly.
	(execute_unwinders): Rename to have a leading underscore.
	(auto_load_packages): Likewise.
	(global scope): Adjust call to auto_load_packages accordingly.
	(GdbSetPythonDirectory): Likewise.
	* python/py-unwind.c (pyuw_sniffer): Call _execute_unwinders
	instead of execute_unwinders.

gdb/testsuite/ChangeLog:

2019-08-15  Christian Biesinger  <cbiesinger@google.com>

	* gdb.python/python.exp: Expect a leading underscore on
	GdbOutput{,Error}File.