emulators/virtualbox-ose-additions*: Flavorize, add xlibre flavor

Flavors: xorg (default), xlibre, nox11.

While here add PAE option for ARCH=i386, fix warnings from portclippy,
make non-functional changes related to indentation and formatting,
make simplification of CONFLICTS_INSTALL, make unification.

PR: 294806
Sponsored by: UNIS Labs
Co-authored-by: b-aaz <b-aazbsd@proton.me>