Add PUPMODE 12 and quiet operation support in sfs_load.overlay

[dimkr]

This PR adds support for:

• -q: assume yes to all questions
• -c: hide the "Loading x" splash
• +/path/to/sfs to load a SFS if not loaded already
• -/path/to/sfs to unload a SFS if loaded
• /path/to/sfs to load or unload a SFS, depending on current state
• +name.sfs to load a SFS in PSUBDIR or root if not loaded already
• -name.sfs to unload a SFS in PSUBDIR or root if loaded
• name.sfs to load or unload a SFS, depending on current state

The idea is not to complicate sfs_load.overlay (230 lines) and make it impossible to maintain like sfs_load (2527 lines), but to allow the use case of sfs_load -q in a ~/Startup script.

EDIT: also added PUPMODE 12 support with protection against power failure and leftover symlinks.