Search in path order not in extension order. That is prefer exec.bat to
exec.exe if exec.bat is earlier in the path.

On Windows only append extensions if the executable has none. (That's
how the shell works.)

Change-Id: I4774f47e5d8e71efccbce49f3facc360eb2a193e
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>