Fixed filename conversion issues. Specifically:
- Correctly handle '%' when preservation is OFF.
- Accept 4-character extensions in '-ee' without risk of buffer overflow.
- Fixed broken assert when converting long %xx names.
- Store "AUX" as "%00AUX" when preserving names under Win32 (vs. "_AUX").
- Always store files with ':' as path separator.
- Recognize that some Win32 variants (Win2K and later at the least) will
accept both '/' and '\' as pathname separators.
- Correctly convert ".//foo" to "foo" instead of "/foo".
Corrected definition of F_OK under Win32.
