fb_py_win_main.c: fix File Not Found errors on windows
fb_py_win_main.c: fix File Not Found errors on windows

Summary:
This commit resolves an issue with our zipapp executables
on Windows that meant that the only reliable way to start them was
to use the fully path to the executable.

The root cause is that the __wargv array is produced by parsing the
process command line into an array, and if you ran `watchman-wait -h`
__wargv[0] would have `watchman-wait` rather than the fully qualified
path to the executable that the zipapp plumbing requires.

The fix is to ask the system for the fully qualified path and ensure that
that gets set as both argv[0] AND argv[1].

Reviewed By: xavierd

Differential Revision: D21190350

fbshipit-source-id: eeb95084592d30a028a93b2b03877f8cc6c72729

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift