Fix gdb-7.6 crash in windows

Fix a call to Py_SetProgramName() with uninitialized progname, which
doesn't crash on linux (probably by accident) and work fine as long
as PYTHONHOME is correctly set.  It crashes in windows, however.

This CL initializes progname and not calls Py_SetProgramName() immediately
(as originally intended) to allow IS_PY3K to take effect if gdb is later
compiled with python-3.x

Also add prototypes for new functions to stop -Werror turns warning
into error.

Change-Id: I18bbfdb6568602b1da6ecd24e5cc09f1bcde2a19