Various fixes in WIN32-specific code

Various fixes in WIN32-specific code

* Avoid potential buffer overflow in add_exe_ext_if_no_to_fullpath.
* Avoid using zero-padding strncpy function.
* Removed superfluous newline characters from log messages.
* For consistency, moved variable declarations to the top of the scope
  to please older compilers.
* For consistency, used C89-style comments to please older compilers.
* Fixed source code formatting.